2ool: 


BIOLOGY 
LIBRARY 

G 


GENERAL    ZOOLOGY 

PRACTICAL,    SYSTEMATIC 
AND    COMPARATIVE 


BEING  A   REVISION   AND    REARRANGEMENT   OF 

ORTON'S  COMPARATIVE  ZOOLOGY 
M 


BY 


CHARLES    WRIGHT   DODGE,   M.S. 

PROFESSOR  OF  BIOLOGY   IN  THE  UNIVERSITY  OF  ROCHESTER 


NEW   YORK  • :  •  CINCINNATI  - : .  CHICAGO 

AMERICAN    BOOK    COMPANY 


07 


BIOLOGY 
UBRARY 

G 


COPYRIGHT,  1876,  1883,  1894,  BY  HARPER  &  BROTHERS. 

COPYRIGHT,  1903,  BY  AMERICAN  BOOK  COMPANY. 

ENTERED  AT  STATIONERS'  HALL,  LONDON. 


GEN.    ZOOLOGY. 
W.  P    I 


PREFACE 

IN  preparing  this  text-book  of  General  Zoology  an 
attempt  has  been  made  to  meet  the  wants  of  teachers 
who  desire  a  treatment  of  the  subject  somewhat  dif- 
ferent from  that  contained  in  earlier  editions  of  Pro- 
fessor Orton's  work,  and  to  furnish  a  course  of  study 
suited  to  the  needs  of  the  general  student  who  wishes 
to  learn  the  principal  facts  and  theories  of  zoology  and 
thus  to  obtain  a  fairly  comprehensive  idea  of  the  sci- 
ence. To  this  end  it  has  seemed  desirable  so  to 
arrange  a  course  of  study  that  the  student  may  gain 
by '  personal  observation  concrete  knowledge  of  the 
structure  and  activities  of  animals,  and,  by  so  doing, 
acquire  some  familiarity,  slight  perhaps,  but  neverthe- 
less valuable,  with  the  method  of  zoological  investi- 
gation ;  that  he  may  obtain  also  a  knowledge  of  the 
relationships  of  animals  as  expressed  in  an  accepted 
scheme  of  classification ;  that  he  may,  further,  broaden 
this  knowledge  by  a  comparison  of  animals  in  their 
structural  and  physiological  relationships ;  and  that, 
finally,  he  be  placed  in  position  to  understand  the  sig- 
nificance of  the  more  important  theories  of  the  science. 
With  these  aims  in  view  the  text  of  Professor  Orton's 
"  Comparative  Zoology "  has  been  revised  and  rear- 
ranged as  described  below. 

3 


326085 


4  PREFACE 

The  pedagogical  importance  of  laboratory  and  field 
study  has  led  to  the  introduction  of  a  series  of  exercises 
upon  the  structure,  physiology,  and  habits  of  represen- 
tative animals.  These  exercises  suggest  the  more  im- 
portant topics  for  study  rather  than  give  an  inflexible 
outline  to  be  followed  in  detail.  The  teacher  is  thus 
left  free  to  adapt  and  modify  the  laboratory  course  to 
suit  the  peculiar  needs  of  his  classes  and  his  equip- 
ment. The  exercises  lead  to  the  study  of  Systematic 
Zoology,  to  which  they  serve  as  the  natural  introduc- 
tion, the  classification  of  animals  being  based  upon 
their  structural  relationships.  With  the  anatomy  of 
the  typical  forms  examined  in  the  practical  exercises 
in  mind,  the  student  ought  to  have  no  trouble  in  under- 
standing the  structural  modifications  mentioned  in  the 
descriptions  of  the  principal  classes  and  orders.  Hav- 
ing thus  enlarged  his  view  of  the  animal  kingdom,  he 
is  in  position  to  appreciate  the  elementary  facts  of 
Comparative  Zoology  and  to  understand  the  main  fea- 
tures of  the  current  zoological  theories.  Believing  this 
to  be  a  logical  sequence  of  study,  the  book  has  been 
arranged  in  accordance  therewith.  With  the  exception 
of  slight  changes,  the  laboratory  exercises  are  the  same 
as  those  recommended  by  the  New  York  State  Science 
Teachers'  Association.  The  system  of  classification 
adopted  is  that  given  by  Parker  and  Haswell  in  their 
"  Text-book  of  Zoology,"  a  work  which  will  long  be  a 
standard  of  reference  for  teachers  in  secondary  schools. 
Part  I  and  Part  II  of  Professor  Orton's  book  have  been 
transposed  so  as  to  place  classification  before  the  dis- 


PREFACE  5 

cussion  of  Comparative  Zoology.  The  addition  of  a 
chapter  on  "  The  Origin  of  Animal  Species  "  will,  it  is 
hoped,  enable  the  student  to  understand  the  most  im- 
portant, at  least,  of  zoological  theories.  A  number  of 
new  figures  have  been  incorporated.  An  asterisk  at 
the  head  of  a  chapter  indicates  that  the  subject-matter 
of  the  chapter  may  be  illustrated  by  practical  work, 
for  which  directions  will  be  found  in  the  Appendix. 

Acknowledgments  are  due  to  Messrs.  D.  Appleton 
and  Company  for  permission  to  reproduce  Figures 
39(11)  and  367  from  Thomson's  "Outlines  of  Zoology  "  ; 
to  the  J.  B.  Lippincott  Company  for  permission  to 
reproduce  Figures  207,  208,  212,  and  256  from  Piersol's 
"  Normal  Histology,"  and  Figure  368  from  Smith's 
"  Economic  Entomology  "  ;  to  Messrs.  W.  B.  Saunders 
and  Company  for  permission  to  reproduce  Figure  203 
from  G.  C.  Huber's  edition  of  Bohm  and  Davidoff's 
"Text-book  of  Histology,"  and  to  Professor  Alfred 
Schaper  of  the  University  of  Breslau,  for  permission  to 
reproduce  Figure  211  from  his  edition  of  Stohr's  "Text- 
book of  Histology." 

CHARLES    WRIGHT   DODGE. 
UNIVERSITY  OF  ROCHESTER. 


The  first  thing  to  be  determined  about  a  new  specimen  is  not  its  name 
but  its  most  prominent  character.  Until  you  know  an  animal,  care  not  for 
its  name.  —  AGASSIZ. 

The  great  benefit  which  a  scientific  education  bestows,  whether  as 
training  or  as  knowledge,  is  dependent  upon  the  extent  to  which  the  mind 
of  the  student  is  brought  into  immediate  contact  with  facts  —  upon  the 
degree  to  which  he  learns  the 'habit  of  appealing  directly  to  Nature.— 
HUXLEY. 


CONTENTS 


INTRODUCTION 

PAGE 

Definition  of  Zoology,  and  its  Place  among  the  Sciences  .         .         .       1 1 
Historical  Sketch  i  c 


PART    I 

STRUCTURAL   AND   SYSTEMATIC  ZOOLOGY 

CHAPTER   I 

PRACTICAL  ZOOLOGY 21 

-  CHAPTER  II 

THE  CLASSIFICATION  OF  ANIMALS 47 

Protozoa 54 

Porifera      . 65 

Ccelenterata 68 

Platyhelminthes 82 

Nemathelminthes 84 

Trochelminthes 85 

Molluscoida        .         .         .         .         .         .         .  .         -85 

Echinodermata 87 

Annulata .         .         -95 

Arthropoda 97 

Mollusca 124 

Chordata 137 

CHAPTER   III 

SYSTEMATIC  ARRANGEMENT  OF  REPRESENTATIVE  FORMS         .        .  202 

7 


8  CONTENTS 

PART    II 

COMPARATIVE   ZOOLOGY 
CHAPTER   IV 

PAGE 

MINERALS  AND  ORGANIZED  BODIES  DISTINGUISHED        .        .        .215 

CHAPTER  V 
PLANTS  AND  ANIMALS  DISTINGUISHED    .        ...        .        .217 

CHAPTER  VI 
RELATION  BETWEEN  MINERALS,  PLANTS,  AND  ANIMALS       '  .  ,     .    224 

CHAPTER  VII 
LIFE          .        ,'.--,;..        .        ...        .        .225 

CHAPTER  VIII 

ORGANIZATION  .        .        .        .  r     .     ^  .        .  .  .  .  .    227 

1.  Cells      .         .        .        .         .        .  .  .  ,  .228 

2.  Tissues .  .  .  .     229 

3.  Organs  and  their  Functions  ...  .  .  .  .     240 

CHAPTER   IX 
NUTRITION        .        .        .        .        .        .        .        .        .        .        .    244 

CHAPTER  X 
THE  FOOD  OF  ANIMALS  . 247 

CHAPTER   XI 

How  ANIMALS  EAT 250 

1.  The  Prehension  of  Food 250 

2.  The  Mouths  of  Animals          . 256 

3.  The  Teeth  of  Animals 265 

4.  Deglutition,  or  How  Animals  Swallow  .         .         .        .         .  274 


CONTENTS  9 

CHAPTER  XII 

PAGE 

THE  ALIMENTARY  CANAL 276 

CHAPTER  XIII 
How  ANIMALS  DIGEST 294 

CHAPTER   XIV 
THE  ABSORBENT  SYSTEM 297 

CHAPTER  XV 
THE  BLOOD  OF  ANIMALS         .        . 301 

CHAPTER   XVI 
THE  CIRCULATION  OF  THE  BLOOD 308 

CHAPTER  XVII 
How  ANIMALS  BREATHE 318 

A 

CHAPTER   XVIII 
SECRETION  AND  EXCRETION 329 

CHAPTER   XIX 
THE  SKIN  AND  SKELETON 335 

CHAPTER   XX 

How  ANIMALS  MOVE       ..........     363 

1.  Muscle 364 

2.  Locomotion 366 

CHAPTER  XXI 

THE  NERVOUS  SYSTEM 376 

1.  The  Senses 386 

2.  Instinct  and  Intelligence       .         .         .         .         .         .         .  395 

3.  The  Voices  of  Animals 399 


10  CONTENTS 


CHAPTER   XXII 

PAGE 

REPRODUCTION 402 


CHAPTER   XXIII 

DEVELOPMENT  .        ....        .        .    -  .        .        .        .  409 

1.  Metamorphosis      .         .         .         ....         .         .  419 

2.  Alternate  Generation  • .  .       .         ...         ...         .         .  424 

3.  Growth  and  Repair        .         . 425 

4.  Likeness  and  Variation          •.        .         .    _    .         .   -  .  427 

5.  Homology,  Analogy,  and  Correlation     .         .      '  .         .         .  429 

6.  Individuality          .         .    -     .    •    '..  .         .         .         .  432 

7.  Relations  of  Number,  Size,  Form,  and  Rank  .         .         .  433 

8.  The  Struggle  for  Life 438 

CHAFFER   XXIV 

THE  DISTRIBUTION  OF  ANIMALS      .        .        ....        .        .        .  440 

CHAPTER    XXV 

THE  ORIGIN  OF  ANIMAL  SPECIES    .''..'...        .        .        .        .  450 

NOTES     ...        .        .        .  '';• 467 

THE   NATURALIST'S   LIBRARY         .        .        .         .        .         .483 

APPENDIX,.        .        .        .         .        .        .        .        .        .         .485 

INDEX  .        .        .  •  •   .  r 497 


INTRODUCTION 

i.  Definition  of  Zoology,  and  its  Place  among  the 
Sciences.  —  The  province  of  Natural  History  is  to  de- 
scribe, compare,  and  classify  natural  objects.  These 
objects  have  been  divided  into  the  "organic"  and -the 
"  inorganic,"  or  those  which  are,  and  those  which  are 
not,  the  products  of  life.  Biology  is  the  science  of  the 
former,  and  Mineralogy  the  science  of  the  latter. 
Biology  again  separates  into  Botany,  or  the  Natural 
History  of  Plants,  and  Zoology,  or  the  Natural  History 
of  Animals ;  while  Mineralogy  divides  into  Mineralogy 
proper,  the  science  of  mineral  species,  and  Lithology, 
the  science  of  mineral  aggregates  or  rocks.  Geology  is 
that  comprehensive  knowledge  of  the  earth's  structure 
and  development  which  rests  on  the  whole  doctrine  of 
Natural  History. 

If  we  examine  a  piece  of  chalk,  and  determine  its 
physical  and  chemical  characters,  its  mode  of'  occurrence 
and  its  uses,  so  as  to  distinguish  it  from  all  other  forms 
of  matter,  we  have  its  Mineralogy.  But  chalk  occurs 
in  vast  natural  beds ;  the  examination  of  these  masses 
—  their  origin,  structure,  position,  and  relation  to  other 
rocks  —  is  the  work  of  the  Lithologist.  Further,  we 
observe  that  while  chalk  and  marble  are  chemically 
alike,  they  widely  differ  in  another  respect.  Grinding 
a  piece  of  chalk  so  thin  that  we  can  see  through  it,  and 
putting  it  under  a  microscope,  we  find  imbedded  in  it 
innumerable  bodies,  about  the  hundredth  of  an  inch  in 
diameter,  having  a  well-defined,  symmetrical  shape,  and 
chambered  like  a  nautilus.  We  cannot  say  these  are 

ii 


12  INTRODUCTION 

accidental  aggregations,  nor  are  they  crystals ;  if  the 
oyster  shell  is  formed  by  an  oyster,  these  also  must  be 
the  products  of  life.  Indeed,  the  dredge  brings  up  simi- 
lar microscopic  skeletons  from  the  bottom  of  the  Atlantic. 
So  we  conclude  that  chalk  is  but  the  dried  mud  of  an 
ancient  sea,  the  cemetery  of  countless  animals  that  lived 
and  died  long  ago.  The  consideration  of  their  fossil 
remains  belongs  to  Paleontology,  or  that  part  of  Biology 
which  describes  the  relics  of  extinct  forms  of  life.  To 
study  the  stratigraphical  position  of  the  chalk  bed,  and 
by  the  aid  of  its  Paleontology  to  determine  its  age  and 
part  in  the  world's  history,  is  the  business  of  Geology. 

Of  all  the  sciences,  Zoology  is  the  most  extensive.  Its 
field  is  a  world  of  varied  forms  —  hundreds  of  thousands 
in  number.  To  determine  their  origin  and  development, 
their  structure,  habits,  distribution,  and  mutual  relations 
is  the  work  of  the  Zoologist.  But  so  many  and  far- 
reaching  are  the  aspects  under  which  the  animal  creation 
may  be  contemplated,  that  the  general  science  is  beyond 
the  grasp  of  any  single  person.  Special  departments 
have,  therefore,  arisen ;  and  Zoology,  in  its  comprehen- 
sive sense,  is  the  combined  result  of  the  labors  of  many 
workers,  each  in  his  own  line  of  research. 

Structural  Zoology  treats  of  the  organization  of  animals. 
There  are  two  main  branches :  Anatomy,  which  con- 
siders the  constitution  and  construction  of  the  animal 
frame  ;  and  Physiology,  which  is  the  study  of  the  appara- 
tus in  action.  The  former  is  separated  into  Embryology, 
or  an  account  of  the  successive  modifications  through 
which  an  animal  passes  in  its  development  from  the 
egg  to  the  adult  state ;  and  Morphology,  which  includes 
all  inquiries  concerning  the  form  of  mature  animals,  or 
the  form  and  arrangement  of  their  organs.  The  micro- 
scopical examination  of  any  part,  especially  the  tissues, 
belongs  to  Histology.  Comparative  Zoology  is  the  com- 


INTRODUCTION  13 

parison  of  the  anatomy  and  physiology  of  all  animals, 
existing  and  extinct,  to  discover  the  fundamental  like- 
ness underneath  the  superficial  differences,  and  to  trace 
the  adaptation  of  organs  to  the  habits  and  spheres  of 
life.  It  is  this  comparative  science  which  has  led  to 
such  grand  generalizations  as  the  unity  of  structure 
amidst  the  diversity  of  form  in  the  animal  creation, 
and  by  revealing  the  degrees  of  affinity  between  species 
has  enabled  us  to  classify  them  in  natural  groups,  and 
thus  laid  the  foundation  of  Systematic  Zoology.  When 
the  study  of  structure  is  limited  to  a  particular  class  or 
species  of  animals,  or  to  a  particular  organ  or  part, 
monographic  sciences  are  created,  as  Ornithotomy,  or 
anatomy  of  birds ;  Osteology,  or  the  science  of  bones ; 
and  Odontography,  or  the  natural  history  of  teeth. 

Systematic  Zoology  is  the  classification  or  grouping  of 
animals  according  to  their  structural  and  developmental 
relations.  The  systematic  knowledge  of  the  several 
classes,  as  Insects,  Reptiles,  and  Birds,  has  given  rise 
to  subordinate  sciences,  like  Entomology,  Herpetology,  or 
Ornithology -,1  * 

Distributive  Zoology  is  the  knowledge  of  the  successive 
appearance  of  animals  in  the  order  of  time  (Paleontology 
in  part),  and  of  the  geographical  and  physical  distribu- 
tion of  animals,  living  or  extinct,  over  the  surface  of  the 
earth. 

Theoretical  Zoology  includes  those  provisional  modes  of 
grouping  facts  and  interpreting  them,  which  still  stand 
waiting  at  the  gate  of  science.  They  may  be  true,  but 
we  can  not  say  that  they  are  true.  The  evidence  is 
incomplete.  Such  are  the  theories  which  attempt  to 
explain  the  origin  of  life  and  the  origin  of  species. 

Suppose  we  wish  to  understand  all  about  the  horse. 
Our  first  object  is  to  study  its  structure.  The  whole 

*  See  Notes  at  the  end  of  the  volume. 


14  INTRODUCTION 

body  is  inclosed  within  a  hide,  a  skin  covered  with 
hair ;  and  if  this  hide  be  taken  off,  we  find  a  great  mass 
of  flesh  or  muscle,  the  substance  which,  by  its  power  of 
contraction,  enables  the  animal  to  move.  On  removing 
this,  we  have  a  series  of  bones,  bound  together  with 
ligaments,  and  forming  the  skeleton.  Pursuing  our 
researches,  we  find  within  this  framework  two  main  cavi- 
ties :  one,  beginning  in  the  skull  and  running  through 
the  spine,  containing  the  brain  and  spinal  marrow ;  the 
other,  commencing  with  the  mouth,  contains  the  gullet, 
stomach,  intestines,  and  the  rest  of  the  apparatus  for 
digestion,  and  also  the  heart  and  lungs.  Examinations 
of  this  character  would  give  us  the  Anatomy  of  the 
horse,  or,  more  precisely,  Hippotomy.  The  study  of  the 
bones  alone  would  be  its  Osteology ;  the  knowledge  of 
the  nerves  would  belong  to  Neurology.  If  we  examined, 
under  the  microscope,  the  minute  structure  of  the  hair, 
skin,  flesh,  blood,  and  bone,  we  should  learn  its  Histology. 
The  consideration  of  the  manifold  changes  undergone 
in  developing  from  the  egg  to  the  full-grown  animal, 
would  be  the  Embryology  of  the  horse ;  and  its  Mor- 
phology, the  special  study  of  the  form  of  the  adult  ani- 
mal and  of  its  internal  organs. 

Thus  far  we  have  been  looking,  as  it  were,  at  a  steam 
engine,  with  the  fires  out,  and  nothing  in  the  boiler; 
but  the  body  of  the  living  horse  is  a  beautifully  formed, 
active  machine,  and  every  part  has  its  different  work  to 
do  in  the  working  of  that  machine,  which  is  what  we 
call  its  life.  The  science  of  such  operations  as  the 
grinding  of  the  food  in  the  complex  mill  of  the  mouth ; 
its  digestion  in  the  laboratory  of  the  stomach  ;  the  pump- 
ing of  the  blood  through  a  vast  system  of  pipes  over 
the  whole  body ;  its  purification  in  the  lungs ;  the  pro- 
cess of  growth,  waste,  and  repair;  and  that  wondrous 
telegraph,  the  brain,  receiving  impressions,  sending 


INTRODUCTION  1 5 

messages  to  the  muscles,  by  which  the  animal  is  en- 
dowed with  voluntary  locomotion  —  this  is  Physiology. 
But  horses  are  not  the  only  living  creatures  in  the  world; 
and  if  we  compare  the  structures  of  various  animals,  as 
the  horse,  zebra.,  dog,  monkey,  eagle,  and  codfish,  we 
shall  find  more  or  fewer  resemblances  and  differences, 
enough  to  enable  us  to  classify  them,  and  give  to  each 
a  description  which  will  distinguish  it  from  all  others. 
This  is  the  work  of  Systematic  Zoology.  Moreover,  the 
horses  now  living  are  not  the  only  kinds  that  have  ever 
lived;  for  the  examination  of  the  earth's  crust  —  the 
great  burial  ground  of  past  ages  —  reveals  the  bones  of 
numerous  horselike  animals  :  the  study  of  this  preadam- 
ite  race  belongs  to  Paleontology.  The  chronological 
and  geographical  distribution  of  species  is  the  depart- 
ment of  Distributive  Zoology.  Speculations  about  the 
origin  of  the  modern  horse,  whether  by  special  creation, 
or  by  development  from  some  allied  form  now  extinct, 
are  kept-  aloof  from  demonstrative  science,  under  the 
head  of  Theoretical  Zoology. 

2.  History. — The  Greek  philosopher  Aristotle  (384- 
322  B.C.)  is  called  the  "  Father  of  Zoology."  Certainly, 
he  is  its  only  great  representative  in  ancient  times, 
though  his  frequent  allusions  to  familiar  works  on  anat- 
omy show  that  something  had  been  done  before  him. 
His  "  History  of  Animals,"  in  nine  books,  displays  a 
wonderful  knowledge  of  external  and  internal  structure, 
habits,  instincts,  and  uses.  His  descriptions  are  incom- 
plete, but  generally  exact  so  far  as  they  go.  Alexander, 
it  is  said,  gave  him  nine  hundred  talents  to  collect  mate- 
rials, and  put  at  his  disposal  several  thousand  men,  for 
hunting  specimens  and  procuring  information. 

The  Romans  accomplished  little  in  natural  science, 
though  their  military  expeditions  furnished  unrivaled 
opportunities.  Nearly  three  centuries  and  a  half  after 


1 6  INTRODUCTION 

Aristotle,  Pliny  (23-79  A.D.)  wrote  his  "  Natural  His- 
tory." He  was  a  voluminous  compiler,  not  an  observer; 
he  added  hardly  one  new  fact.  He  states  that  his  work 
was  extracted  from  over  two  thousand  volumes,  most  of 
which  are  now  lost. 

During  the  Middle  Ages,  Natural  History  was  studied 
in  the  books  of  the  ancients ;  and  at  the  close  of  the  fif- 
teenth century  it  was  found  where  Pliny  had  left  it,  with 
the  addition  of  many  vague  hypotheses  and  silly  fancies. 
Albertus  Magnus,  of  the  thirteenth  century,  and  Con- 
rad Gesner  and  Aldrovandus,  of  the  sixteenth,  were 
voluminous  writers,  not  naturalists.  In  the  latter  half 
of  the  sixteenth  century  men  began  to  observe  nature 
for  themselves.  The  earliest  noteworthy  researches 
were  made  on  Fishes,  by  Rondelet  (1507-1556)  and 
Belon  (1517-1564)  of  France,  and  Salviani  (1514-1572) 
of  Italy.  They  were  followed  by  valuable  observations 
upon  Insects,  by  Redi  (1626-1698)  of  Italy,  and  Swam- 
merdam  (1637-1680)  of  Holland;  and  toward  the  end 
of  the  same  century,  the  Dutch  naturalist,  Leeuwen- 
hoeck  (1632-1723),  opened  a  new  world  of  life  by  the 
use  of  the  microscope. 

But  there  was  no  real  advance  of  Systematic  Zoology 
till  the  advent  of  the  illustrious  John  Ray  (1628-1705) 
of  England.  His  "Synopsis,"  published  in  1693,  con- 
tained the  first  attempt  to  classify  animals  according  to 
structure.  Ray  was  the  forerunner  of  "the  immortal 
Swede,"  Linnaeus  (1707-1778),  "the  great  framer  of 
precise  and  definite  ideas  of  natural  objects,  and  terse 
teacher  of  the  briefest  and  clearest  expressions  of  their 
differences."  His  chief  merit  was  in  defining  generic 
groups,  and  inventing  specific  names.2  Scarcely  less 
important,  however,  was  the  impulse  which  he  gave  to 
the  pursuit  of  Natural  History.  The  spirit  of  inquiry, 
which  his  genius  infused  among  the  great,  led  to  voy- 


INTRODUCTION  17 

ages  of  research,  which  resulted  in  the  formation  of 
national  museums.  The  first  expedition  was  sent  forth 
by  George  III.  of  England,  in  1765.  Reaumur  (1683- 
1757)  made  the  earliest  zoological  collection  in  France; 
and  the  West  Indian  collections  of  Sir  Hans  Sloane 
(1660-1752)  were  the  nucleus  of  the  British  Museum. 
The  accumulation  of  specimens  suggested  comparisons, 
which  eventually  resulted  in  the  highest  advance  of  the 
science. 

The  brilliant  style  of  Buffon  (1707-1788)  made  Zool- 
ogy popular,  not  only  in  France,  but  throughout  Europe. 
While  the  genius  of  Linnaeus  led  to  classification,  that 
of  Buffon  lay  in  description.  He  was  the  first  to  call 
attention  to  the  subject  of  Distribution.  Lamarck 
(1745-1829)  of  Paris  was  the  next  great  light.  The 
publication  of  his  "  Animaux  sans  Vertebres,"  in  1801, 
was  an  epoch  in  the  history  of  the  lower  animals.  He 
was  also  the  first  prominent  advocate  of  .the  transmuta- 
tion of  species. 

But  the  brightest  luminary  in  Zoology  was  George 
Cuvier  (1769-1832),  a  German,  born  on  French  soil. 
Before  his  time  "  there  was  no  great  principle  of  classi- 
fication. Facts  were  accumulated,  and  more  or  less  sys- 
tematized, but  they  were  not  yet  arranged  according  to 
law ;  the  principle  was  still  wanting  by  which  to  gen- 
eralize them  and  give  meaning  and  vitality  to  the 
whole."  It  was  Cuvier  who  found  the  key.  He  was 
the  first  so  to  interpret  structure  as  to  be  able  from  the 
inspection  of  one  bone  to  reconstruct  the  entire  animal, 
and  assign  its  position.  His  anatomical  investigations 
revealed  the  natural  affinities  of  animals,  and  led  to 
the  grand  generalization,  that  the  most  comprehensive 
groups  in  the  kingdom  were  based,  not  on  special  char- 
acters, but  on  different  plans  of  structure.  Palissy  had 
long  ago  (1580)  asserted  that  petrified  shells  were  of 
DODGE'S  GEN.  ZOOL.  —  2 


1 8  INTRODUCTION 

animal  origin  ;  but  the  publication  of  Cuvier's  "  Memoir 
on  Fossil  Elephants,"  in  1800,  was  the  beginning  of 
those  profound  researches  on  the  remains  of  ancient 
life  which  created  Paleontology.  The  discovery  of  the 
true  relation  between  all  animals,  living  and  extinct, 
opened  a  boundless  field  of  inquiry ;  and  from  that  day 
the  advance  of  Zoology  has  been  unparalleled.  Special 
studies  of  particular  parts  or  classes  of  animals  have  so 
rapidly  developed,  that  the  history  of  Zoology  during 
the  last  fifty  years  is  the  history  of  many  sciences.3 

But  to  Charles  Darwin  more  than  to  any  other  inves- 
tigator is  due  the  credit  for  the  great  mass  of  researches 
which  has  been  accumulated  during  the  last  half  cen- 
tury. The  publication  of  the  "  Origin  of  Species,"  in 
1859,  marks  the  starting-point  of  modern  zoological  re- 
search. Darwin's  statement  of  the  facts  of  evolution 
and  his  theory  of  the  causes  which  produce  species  of 
organisms,  both  plant  and  animal,  attracted  the  atten- 
tion of  all  biologists,  and  now  practically  all  investiga- 
tion in  the  sciences  of  zoology  and  botany  is  carried  on 
in  the  light  of  the  great  principle  of  evolution. 


PART    I 

STRUCTURAL  AND   SYSTEMATIC 
ZOOLOGY 


Facts  are  stupid  things  until  brought  into  connection  with  some  general 
law.  —  AGASSIZ. 

No  man  becomes  a  proficient  in  any  science  who  does  not  transcend 
system,  and  gather  up  new  truth  for  himself  in  the  boundless  field  of 
research.  —  DR.  A.  P.  PEABODY. 

Never  ask  a  question  if  you  can  help  it;  and  never  let  a  thing  go  un- 
known for  the  lack  of  asking  a  question  if  you  can't  help  it.  —  BEECHER. 

He  is  a  thoroughly  good  naturalist  who  knows  his  own  parish  thor- 
oughly.—  CHARLES  KINGSLEY. 


STRUCTURAL    AND    SYSTEMATIC 
ZOOLOGY 

CHAPTER   I 

PRACTICAL   ZOOLOGY 

IT  is  very  desirable  that  the  student  should  get  as 
much  as  possible  of  his  knowledge  of  zoology  from 
a  study  of  the  animals  themselves  rather  than  from 
descriptions.  It  is  of  course  impracticable  as  well  as 
undesirable  to  depend  entirely  upon  this  source  of  infor- 
mation. Nevertheless,  the  student  should  be  taught 
how  to  study  specimens,  both  living  and  dead.  For  this 
reason  the  following  exercises  in  the  practical  examina- 
tion of  animal  forms  have  been  prepared.  They  consist 
mainly  of  mere  suggestions  of  topics  for  study,  the 
details  being  left  to  the  teacher,  for  it  is  recognized  that 
if  a  definite  outline  to  be  followed  rigidly  were  offered, 
it  would  probably  be  too  elaborate  for  those  schools 
where  only  a  few  weeks  can  be  devoted  to  the  subject, 
and  too  meager  for  the  schools  in  which  a  longer  course 
is  given. 

The  exercises  provide  for  a  study  of  the  activities 
and  habits  of  the  living,  as  well  as  an  examination  of- 
the  structure  of  the  dead  specimen.  Every  important 
branch  of  the  animal  kingdom  is  represented  by  at  least 
one  common  and  easily  obtained  example.  It  is  sug- 
gested that  the  example  be  studied  before  a  text  lesson 
is  assigned  on  the  group  which  it  represents.  In  this 
way  the  student  will  have  a  certain  amount  of  original 


22       STRUCTURAL  AND   SYSTEMATIC  ZOOLOGY 

information  which  will  enable  him  more  clearly  to  com- 
prehend the  description  of  related  forms  mentioned  in 
the  text. 

In  every  case  careful  drawings  should  be  made  of 
the  specimen,  and  full  notes  on  its  habits  and  structure 
prepared. 

The  appliances  needed  are  a  scalpel  and  a  pair  of 
forceps,  both  of  medium  size ;  a  magnifying  glass ;  a 
compound  microscope,  if  protozoa  and  other  minute 
forms  are  to  be  studied ;  and  a  small  board  on  which 
larger  specimens  may  be  laid  for,  the  study  of  the  struc- 
ture. If  alcoholic  specimens  are  to  be  studied  they 
may  be  placed  for  examination  in  vegetable  dishes  con- 
taining equal  parts  of  alcohol  and  water  to  prevent 
drying  of  the  parts.  There  should  be  enough  of  the 
mixture  to  cover  the  specimen.  Specimens  which 
have  been  preserved  in  formalin  may  be  examined  in 
water.  For  more  particular  descriptions  of  specimens 
and  methods  of  work  reference  may  be  made  to  the 
laboratory  manuals  and  text-books  mentioned  in  the 
Appendix. 

INVERTEBRATES 
Protozoa 

Amoeba 

Material.  —  More  or  less  uncertainty  usually  attends 
every  attempt  to  provide  at  a  given  time  a  supply  of 
amoebas  for  a  laboratory  class.  Nevertheless,  the  study 
of  this  organism  should  not  on  any  account  be  omitted, 
for  from  no  other  one  is  so  much  to  be  learned  regarding 
the  fundamental  properties  of  living  things.  A  thor- 
ough study  of  the  amoeba  forms  the  basis  of  all  sound 
biological  training. 

Specimens  of  amoeba  are  often  to  be  found  in  the 


PRACTICAL  ZOOLOGY  23 

following  places :  in  the  slime  on  the  under  side  of  lily 
pads  and  along  the  stem  ;  in  the  superficial  layer  of  mud 
in  ponds  and  slowly  flowing  streams;  in  damp  moss 
from  sphagnum  swamps ;  in  the  deposit  on  the  sides  of 
water  barrels  in  greenhouses ;  in  aquaria  which  have 
been  standing  for  some  time  and  which  contain  no  crusta- 
ceans like  DapJmia,  Cypris,  Cyclops,  etc.  In  case  no 
specimens  are  obtained  from  ordinary  sources,  amoeboid 
cells  may  be  used  instead.  These  may  be  found  by 
tearing  to  pieces  the  gills  of  a  clam,  or  a  mussel,  or  by 
killing  a  frog,  cutting  through  the  skin  of  the  abdomen 
or  leg,  and  removing  a  drop  of  the  colorless  fluid  (lymph). 
To  study  the  specimen,  collect  with  a  pipette  a  drop 
of  the  water  supposed  to  contain  amoebas,  or  a  drop  of 
lymph,  place  it  on  a  glass  slide,  put  on  the  cover  glass, 
and  examine  with  a  low  power,  f ,  J,  or  \  inch  objective. 
Be  sure  to  have  some  sediment  or  a  hair  under  the  cover 
glass  in  order  that  the  weight  of  the  latter  may  not 
crush  the  specimen. 

Topics  for  Study.  —  The  shape,  an  irregular  outline, 
changing  as  the  animal  moves  along  (sketch  the  outline 
at  intervals  of  one  or  two  minutes,  and  compare  the 
successive  sketches) ;  the  motion,  note  its  rate  and  direc- 
tion; the  change  of  shape  is  due  very  largely  to  the  pro- 
trusion of  portions  of  the  body  substance  in  the  form  of 
blunt  processes  called  pseudopodia  (singular,  pseudopo- 
dium)  (Fig.  I,  page  57). 

With  a  higher  power  (^  or  \  inch  objective),  examine 
the  animal's  structure,  noting  that  it  is  composed  mainly 
of  a  clear,  semifluid  substance,  — protoplasm,  —  in  which 
are  embedded  numerous  granular  bodies  of  various 
sizes  and  colors,  some  recognizable  as  fragments  of  vege- 
table substance,  together  with,  probably,  one  or  more 
diatoms  or  other  minute  organisms.  In  some  part  of 


24       STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 

the  body  there  will  usually  be  seen  a  large,  clear,  appar- 
ently empty  circle,  which  from  time  to  time  suddenly 
contracts  and  disappears  from  view.  This  is  the  con- 
tractile vacuole,  and  is  supposed  to  be  an  organ  of  excre- 
tion, since  uric  acid,  one  of  the  forms  in  which  nitrog- 
enous waste  material  leaves  the  body  in  higher  animals, 
has  been  found  in  the  vacuoles  of  certain  animals  closely 
related  to  the  amoeba.  The  vacuole,  though  apparently 
disk-shaped,  is  really  spherical.  Closer  examination  will 
show  a  small  round  mass,  usually  slightly  darker  than  the 
rest  of  the  body  and  often  distinctly  and  evenly  dotted 
with  fine  points,  and  surrounded  by  a  plainly  defined 
line.  This  is  the  nucleus  surrounded  by  its  membrane. 
Its  shape  is  constant,  except  when  the  amoeba  is  in  the 
process  of  division.  Still  more  careful  study  will  show 
that  the  body  substance  can  be  rather  sharply  divided 
into  two  regions,  an  outer,  clear,  quite  homogeneous 
portion,  the  ectoplasm,  and  an  inner,  granular  region, 
the  endoplasm. 

If  the  animal  be  watched  for  a  short  time,  it  will  proba- 
bly be  seen  to  ingest  food  particles,  or,  possibly,  capture 
another  animalcule.  In  either  case,  the  mode  of  pro- 
cedure should  be  watched  and  the  fate  of  the  captured 
particle  followed.  Some  of  the  ingested  material  will 
be  disgorged,  while  certain  pieces  will  be  seen  slowly  to 
disintegrate  and  to  disappear  as  they  dissolve  in  the 
droplet  of  water  in  which  the  animal  swallowed  them. 
From  the  behavior  of  these  particles  and  from  the 
changes  seen  to  take  place  in  substances  which  have 
been  given  the  amoeba  for  experimental  purposes,  it  is 
believed  that  the  animal  produces  in  its  body  substances 
analogous  to  the  digestive  juices  of  higher  forms.  The 
disintegration  of  the  food  particles,  then,  indicates  that 
they  are  being  digested.  When  they  have  reached  the 
stage  of  solution,  they  can,  of  course,  no  longer  be  seen. 


PRACTICAL   ZOOLOGY  25 

The  process  of  respiration  cannot  be  followed  in  this 
organism,  there  being  no  definite  organs  analogous  to 
the  gills  and  lungs  of  higher  forms  devoted  to  this 
function ;  but  the  interchange  of  oxygen  and  carbon 
dioxide,  which  is  the  essential  part  of  the  process,  is 
believed  to  take  place  through  the  superficial  part  of 
the  body. 

The  nervous  properties  of  the  animal  are  well  shown 
when  it  comes  in  contact  with  a  foreign  body,  evidence 
for  the  possession  of  the  sense  of  touch  being  easily 
obtained  while  the  movements  are  being  watched. 

The  contractions  of  the  body  substance  show  that  it 
is  muscular. 

In  exceptional  circumstances,  an  amoeba  in  the 
process  of  division,  or  fission,  may  be  found,  the  body 
separating  into  two  parts  connected  at  first  by  a  thread 
of  protoplasm  .which  eventually  breaks,  two  distinct 
organisms  thus  being  formed.  This  process  may  be 
more  easily  studied,  however,  in  Paramecium^  the  "  slip- 
per animalcule."  Preceding  the  division  of  the  single 
cell  composing  the  body,  there  is  a  division  of  its 
nucleus.  Some  of  the  "shelled"  forms,  like  Arcella 
and  Difflugia,  may  be  used  for  comparison. 

Euglena 

Material.  —  Some  of  the  green  scum  which  forms  on 
the  inside  of  aquaria  is  likely  to  yield  abundant  speci- 
mens. If  not,  they  may  usually  be  raised  by  allowing 
some  pieces  of  green-coated  bark,  or  a  portion  of  a 
flowerpot  covered  with  the  green  film  which  forms  on 
the  sides  of  damp  pots,  to  stand  covered  with  water  in 
a  dish  in  a  sunny  place  for  several  days.  Scrape  off 
some  of  the  green  film  in  the  bottom  of  the  dish,  and 
examine  according  to  the  directions  given  for  amoeba. 
The  animal  may  be  recognized  by  its  green  color, 


26       STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

amcebalike  changes  in  the  shape  of  the  body,  and  by 
the  presence  of  a  whiplashlike  organ  (flagellum)  by 
means  of  which  it  propels  itself  through  the  water. 

Topics  for  Study.  —  The  elongated,  highly  flexible 
body,  its  motions  and  color;  the  position  and  move- 
ments of  the  flagellum;  the  red  stigma,  or  "eye  spot," 
and  the  contractile  vacuole,  both  near  the  mouth 

(Fig.  4). 

Paramecium 

Material.  —  The  slipper  animalcule  is  more  readily 
obtained  than  almost  any  other  of  the  protozoa.  There 
are  various  ways  of  raising  it  in  abundance  for  labora- 
tory purposes.  Hay  or  marsh  grass  cut  into  pieces  a 
few  inches  long  may  be  placed  in  a  convenient  dish, 
covered  with  water,  and  set  in  a  warm  room  for  one  or 
two  weeks,  at  the  end  of  which  time  there  will  probably 
have  formed  a  pellicle  on  the  surface.  This  will  con- 
sist largely  of  rod-shaped  or  threadlike  bacteria,  and 
feeding  upon  them  will  be  seen  many  kinds  of  infusoria, 
among  the  latter  being  Stylonychia,  which  may  be  recog- 
nized by  its  large  bristlelike  cilia  and  its  springing 
motions,  and  Paramecium,  the  "slipper  animal,"  covered 
everywhere  with  fine  cilia  and  having  a  more  smooth, 
gliding  movement.  Another  satisfactory  method  of 
procuring  specimens  is  to  place  a  handful  of  water 
plants,  like  Anacharis  (waterweed),  Utricidaria  (bladder- 
wort),  or  Potamogeton  (pond weed)  in  just  enough  water 
to  cover  the  plants,  and  let  the  mass  stand  in  a  warm, 
dark  place  until  decay  begins,  at  which  time  the  water 
will  probably  be  found  to  be  swarming  with  animalcules. 

In  preparing  the  specimens  for  microscopic  examina- 
tion, follow  the  directions  given  for  amoeba.  It  is 
always  well  to  put  under  the  cover  glass  a  few  frag- 
ments of  the  scum  consisting  of  bacteria,  for  the  ani- 
malcules will  gather  around  these  masses  and  remain, 


PRACTICAL   ZOOLOGY  2/ 

feeding  quietly.  Otherwise  their  motions  are  likely  to 
be  so  rapid  that  study  of  the  specimen  may  be  quite 
impossible.  Or,,  a  few  fibers  of  absorbent  cotton  may 
be  placed  in  the  drop  of  water  containing  the  animals, 
thus  forming  meshes  to  entangle  them. 

Aquarium  Study.  —  In  the  culture  note  the  swarms 
of  animalcules,  their  position  with  reference  to  the  sur- 
face of  the  water,  the  sides  of  the  dish,  the  direction  of 
the  brightest  light ;  their  size,  color,  and  movements.  - 

Microscope  Study.  —  With  the  low  power  study  the 
movements,  their  direction  and  rate;  the  flexibility  of 
the  body,  as  seen  when  the  animal  passes  through 
narrow  openings  or  around  corners;  the  definite  shape 
of  the  body  (compare  with  amoeba) ;  the  nervous  prop- 
erties, especially  the  sense  of  touch  exhibited  when  the 
animal  comes  into  contact  with  a  foreign  body ;  the 
tendency  to  collect  around  food  masses  and  air  bub- 
bles, or  near  the  margin  of  the  cover  glass,  the  latter 
tendency  best  seen  if  the  water  is  very  foul;  animals 
in  the  process  of  fission  (resembling  a  single  speci- 
men more  or  less  constricted  in  the  middle  of  the 
body,  Fig.  10),  or  in  conjugation  (two  individuals 
attached  together  by  their  ventral  sides). 

With  the  high  power  the  structure  of  individual  ani- 
mals and  the  functions  of  various  parts  of  their  bodies 
may  be  studied.  Note  the  arrangement,  shape,  size, 
and  movements  of  the  cilia  (their  motions  may  be 
stopped  by  the  application  of  a  drop  of  iodine  solution) ; 
the  presence  of  the  cuticle  (cell  wall) ;  the  mouth  open- 
ing leading  to  the  gullet,  the  latter  lined  by  short  cilia 
whose  motions  cause  a  current  of  water  bearing  food 
particles  to  pas£  down  into  the  body,  where  droplets 
(food  vacuoles)  form  and,  after  reaching  a  fairly  uniform 
size,  are  separated  from  the  end  of  the  gullet  and  carried 
around  through  the  body  by  the  flow  of  the  body  sub- 


28       STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 

stance  (protoplasm) ;  the  position  and  movements  of  the 
contractile  vacuoles,  one  near  each -end  of  the  body,  by 
means  of  which  the  waste  water  is  removed  from  the 
body ;  the  movements  of  the  cilia  near  the  mouth  open- 
ing which  produce  currents  in  the  water,  thus  bringing 
food  particles  within  reach ;  the  trichocysts,  thought  to 
be  organs  of  defense,  lying  parallel  to  one  another  just 
under  the  cuticle  (their  discharge  may  be  produced  by 
running  a  drop  of  acetic  acid  under  the  cover  glass) ; 
the  large  nucleus  may  be  seen  lying  near  the  center  of 
the  body  after  the  application  of  a  drop  of  acetic  methyl 
green  or  of  methylene  blue  (Fig.  9). 

Evidence  of  the  possession  of  nervous  properties  will 
be  seen  in  the  animal's  extreme  sensitiveness  to  contact 
with  foreign  bodies,  in  its  selection  of  food,  which  con- 
sists almost  entirely  of  bacteria,  in  its  tendency  to 
collect  on  the  lighter  side  of  the  aquarium  jar  when 
the  latter  stands  remote  from  the  window. 

Attention  should  be  called  to  the  possible  means  of 
dispersal  of  the  animal,  to  its  value  as  a  scavenger,  and 
particularly  to  the  "physiological  division  of  labor" 
among  the  various  portions  of  its  body,  which,  though  a 
single  cell,  has  its  parts  very  plainly  adapted  to  perform 
many  different  functions. 

Vorticella 

Material.  —  Specimens  are  usually  to  be  found  attached 
to  the  sides  of  the  aquarium  in  which  Paramecium  has 
been  raised,  or  to  the  fragments  of  hay,  jwater  plants, 
etc.,  therein.  Prepare  the  specimens  according  to  the 
directions  for  Amoeba. 

Topics  for  Study.  —  With  the  low  power  study  the 
shape  of  the  animal,  consisting  of  the  body  portion  and 
the  flexible  stalk  ;  the  movements  of  each  part,  especially 
the  coiling  of  the  stalk ;  the  position  and  movements  of 


PRACTICAL   ZOOLOGy  2Q 

the  cilia,  note  that  the  latter  are  confined  to  the  margin 
of  the  bell-shaped  body  ;  the  miniature  vortex  into  which 
food  particles  are  drawn  by  the  action  of  the  cilia. 

With  the  high  power,  study  the  position  and  shape 
of  the  large  crooked  nucleus,  the  motions  of  the  single 
contractile  vacuole,  the  structure  of  the  stalk,  and  the 
ingestion  of  food  particles  (Fig.  1 1,  b\  Some  specimens 
will  probably  be  found  in  the  state .  of  fission.  The 
early  stages  may  be  identified  by  the  broadening  of  the 
body  transversely,  the  absence  of  cilia,  and  by  a  vertical 
groove  indicating  the  direction  of  division.  Later  the 
two  parts  become  more  or  less  completely  separated, 
one  having  a  circle  of  cilia  around  its  lower  portion.  A 
few  minutes  after  the  cilia  are  formed  the  animalcule 
breaks  away  from  its  companion,  leaving  the  latter  in 
possession  of  the  stalk,  and  swims  away  by  means  of  its 
temporary  locomotor  cilia  to  select  a  site  for  attaching 
itself  and  developing  its  own  stalk  (Fig.  1 1,  a). 

Conjugation  may  sometimes  be  observed,  and  may  be 
recognized  by  the  fact  that  a  large  stalked  individual 
has  attached  to  the  lower  portion  of  its  body  a  much 
smaller  nonstalked  individual,  which  gradually  merges 
into  the  body  of  the  larger  animal  and  disappears.  The 
nuclei  of  the  two  individuals  fuse  together  and  fertiliza- 
tion is  thus  accomplished,  but  the  changes  which  take 
place  within  the  two  cells  during  the  process  of  conjuga- 
tion can  be  demonstrated  only  upon  specimens  especially 
prepared  for  this  purpose,  the  method  of  preparation 
being  too  intricate  for  beginners. 

Metazoa 

Porifera 

Material.  — Simple  marine  sponges  maybe  obtained  of 
dealers  in  laboratory  supplies.  ,Fresh-water  sponges, 
Spongilla  (green)  and  Myenia  (brown)  are  to  be  looked 


30       STRUCTURAL  AND    SYSTEMATIC    ZOOLOGY 

for  in  clear  water  attached  to  submerged  branches,  logs, 
and  rocks,  and  especially  on  the  timbers  of  dams  and 
mill  races.  In  purchasing  toilet  sponges  for  specimens, 
care  should  be  taken  to  select  some  which  show  single, 
others  numerous,  openings  and  canals,  while  still  others 
should  have  particles  of  sand  and  of  shells  embedded  in 
the  lower  part. 

Using  Grantia  as  a  type  of  the  simple  sponge,  study 
its  shape,  color,  and  mode  of  attachment;  the  large 
opening  (osculum)  at  the  upper  end  surrounded  with  a 
row  of  spicules.  Note  the  small  openings,  inhalant  pores, 
on  the  surface.  Cut  the  sponge  open  longitudinally  and 
note  the  pores  opening  into  the  central  cavity.  These 
pores  will  be  seen  to  be  the  ends  of  canals  which  run 
horizontally  outward.  They  do  not,  however,  open  to 
the  outer  surface.  These  are  the  radial  canals.  Lying 
between  two  adjacent  radial  canals  will  be  found  an 
incurrent  canal,  the  outer  opening  of  which  is  on  the 
outer  surface  of  the  sponge.  This  canal  has  no  opening 
directly  into  the  central  cavity.  Water  carrying  food 
particles  is  drawn  into  the  incurrent  canals  and  passes 
into  the  radial  canals  through  pores  in  the  walls  of 
tissue  between  the  two  canals.  It  then  passes  out  of  the 
pores  at  the  inner  ends  of  the  radial  canals,  into  the  larger 
central  cavity,  and  out  through  the  osculum.  The  flow 
of  water  is  produced  by  the  action  of  ciliated  cells  which 
line  the  radial  canals  (Figs.  13,  14). 

Microscopic  sections  will  show  the  arrangement  of  the 
canals  and  of  the  spicules  of  lime  in  the  tissue  of  the 
sponge,  as  well  as  the  arrangement  of  the  cellular  parts 
of  the  body.  Large  amoeboid  cells  (ova)  are  frequently 
found  in  the  walls  between  the  canals.  The  spicules 
may  be  obtained  free  from  adhering  tissue  by  boiling  a 
fragment  of  Grantia  in  caustic  potash  in  a  test  tube. 
As  the  spicules  do  not  dissolve,  the  fluid  may  be  drained 


PRACTICAL   ZOOLOGY  31 

off  and  a  drop  of  the  sediment  in  the  tube  placed  under 
the  microscope  for  examination.  If  a  fragment  of 
Grantia  be  placed  in  'weak  acetic  acid,  an  abundant 
effervescence  will  take  place,  giving  evidence  that  the 
spicules  are  composed  of  carbonate  of  lime,  the  rest  of 
the  sponge  body  remaining  undissolved. 

In  studying  the  toilet  sponge,  note  its  color,  shape, 
weight,  and  elasticity ;  study  the  position  and  arrange- 
ment of  the  large  and  small  canals ;  note  the  embedded 
sand  particles,  shells,  etc. ;  the  texture  of  various  speci- 
mens; put  a  fragment  under  the  low  power  of  the 
microscope  and  note  how  the  fibers  are  arranged ;  soak 
a  sponge  in  water  and  measure  the  amount  held  in  the 
meshes  by  squeezing  it  out  into  a  graduate. 

Fresh-water  sponges  are  not  easily  kept  alive  in  the 
laboratory  nor  is  their  structure  very  plain.  Their  mode 
of  growth,  branching,  color,  and  friable  texture  may  be 
studied.  With  a  magnifying  glass  numerous  pores  will 
be  seen  on  Spongilla,  while  the  oscula  of  Myenia  are 
plainly  visible.  Microscopic  sections  will  show  the 
double-pointed,  flinty  spicules  traversing  the  tissues  in 
all  directions.  Small  spherical,  seedlike  gemmules  may 
be  obtained  in  the  older  part  of  the  sponge  in  the  fall, 
and  will  "  germinate  "  in  a  few  days  if  kept  undisturbed 
in  a  dis4i  of  water.  Only  very  little  growth  is  likely  to 
take  place. 

Coelenterata 
Hydra 

Material.  —  Either  the  green  or  the  brown  species 
may  be  used ;  the  latter,  being  much  the  larger,  is 
preferable.  It  will  be  found  attached  to  the  stems  of 
water  plants  which  may  be  kept  in  aquarium  jars. 
The  animals  will  often  migrate  to  the  sides  of  the 
jar,  where  they  can  be  studied  with  or  without  a  lens. 


32        STRUCTURAL   AND    SYSTEMATIC    ZOOLOGY 

The  green  species  is  common  on  species  of  Vaucheria 
or  greenfelt,  which  grow  in  rapidly  flowing  creeks. 
Mats  of  the  plant  may  be  put  into  white  earthenware 
dishes.  After  a  few  minutes  the  hydras  will  expand 
and  be  easily  seen  against  the  background  formed 
by  the  dish. 

Aquarium  Study. — With  the  naked  eye  or  with  a 
lens  study  the  hydra  in  situ,  noting  its  color,  shape, 
size,  the  body  and  the  tentacles,  the  number  and 
extensibility  of  the  latter;  touch  the  body  or  the  ten- 
tacles with  a  bristle  and  note  the  sensitiveness  of  the 
animal.  Look  for  individuals  bearing  buds ;  the  num- 
ber and  position  of  the  latter;  the  radial  symmetry 
of  the  body.  Note  how  well  the  shape  and  color 
of  the  animal  adapt  it  to  its  surroundings  (Fig.  17). 
'  Microscope  Study.  —  Transfer  a  fragment  of  the 
plant  bearing  a  hydra  to  a  watch  glass  (or,  if  the 
animal  is  fastened  to  the  side  of  the  jar,  detach  it 
with  a  pipette),  and  examine  under  a  lens  or  under 
a  low  power.  Note  again  the  movements  of  the 
body  and  tentacles.  .  Put  a  minute  fragment  of  fresh 
meat  within  reach  of  the  tentacles  and  endeavor  to 
see  the  hydra  catch  and  swallow  it.  Study  the  move- 
ments of  the  mouth. 

Mount  a  specimen  under  a  cover  glass  arM  study 
the  structure  of  the  body,  its  walls  consisting  of  two 
layers  of  cells ;  the  central  cavity  with  one  exterior 
opening ;  the  color  of  the  inner  cell  layer  and  its 
cause ;  the  structure  of  the  tentacles,  the  groups  of 
nettle  cells;  cause  the  discharge  of  the  latter  by  run- 
ning a  drop  of  weak  acetic  acid  under  the  cover  glass. 
On  individuals  bearing  buds  study  the  structure  and 
actions  of  the  latter ;  their  mode  of  attachment  to 
the  parent ;  the  small  "  colony "  formed  by  this  mode 
of  reproduction  (Fig.  18). 


PRACTICAL  ZOOLOGY  33 

Note  the  effect  of  light  on  hydras  by  putting  several 
in  a  jar  of  water  and  covering  it  with  an  opaque  paper 
through  which  on  one  side  a  hole  one  inch  in  diameter 
has  been  made,  the  jar  then  being  placed  near  a  win- 
dow and  the  hole  being  directed  toward  the  light. 

For  comparison  use  the  sea  anemone,  Metridium 
(Fig.  236).  , 

Campanularian  Hydroid 

Material.  —  Specimens  of  Eucope  or  Obelia  are  found 
attached  to  seaweed  or  submerged  timbers,  below  low 
tide  mark,  in  the  sea.  The  colonies  are  usually  grayish 
in  color,  much  branched,  and  have  a  noticeably  plant- 
like  aspect.  If  living  specimens  are  available  for 
study,  they  may  be  placed  in  small  dishes  of  fresh 
sea  water  and  examined  with  a  magnifying  glass. 
The  various  motions  of  the  polyps  may  be  studied, 
their  protrusion  from  and  withdrawal  into  the  pro- 
tective cup  which  surrounds  each  one,  the  rapid  ex- 
tension and  twisting  of  the  tentacles,  the  protrusion 
of  the  funnel-like  mouth  to  ingulf  particles  of  food 
(minute  scraps  of  fresh  meat  are  suitable),  the  sensi- 
tiveness of  the  various  members  of  the  colony  to 
jarring,  touching  with  a  bristle  point,  agitation  of  the 
water,  and  so  on  (Fig.  20). 

It  will  be  noticed  that  the  colony  consists  of  two 
forms  of  zooid,  one  bearing  tentacles,  the  nutritive 
zooids ;  the  other  being  without  tentacles,  elongated 
in  shape,  and  containing  a  number  of  rounded  bodies. 
This  form  is  the  reproductive  zooid,  and  the  contained 
bodies  are  the  medusa  buds  or  medusoids,  which,  when 
mature,  are  liberated  and  produce  the  eggs  from  which 
new,  branched  colonies  arise.  The  phenomenon  of 
"alternation  of  generations"  is  here  very  marked. 

The  microscopic  structure  is  best  seen  in  mounted 
specimens,  which  may  be  obtained  from  dealers. 
DODGE'S  GEN.  ZOOL.  —  3 


34       STRUCTURAL   AND    SYSTEMATIC    ZOOLOGY 

Platyhelminthes  and  Nemathelminthes 
Tapeworm,  or  Trichina 

Material.  —  Alcoholic  specimens  of  the  former  and 
microscopic  preparations  of  the  latter  may  be  studied 
and  attention  called  to  their  complicated  developmental 
history,  and  their  pathologic  significance  (Figs.  37,  38,  39). 

Echinodermata 

Starfish 

Aquarium  Study-  —  If  live  specimens  can  be  obtained, 
study  their  mode  of  locomotion ;  the  flexibility  of  the 
rays  and  the  body;  the  movement  of  the  spines  along 
the  grooves,  around  the  mouth  opening,  and  at  the 
tip  of  the  ray  where  the  eye  is  located ;  note  the  sensi- 
tiveness of  the  various  parts,  particularly  of  the  tube 
feet*  and  of  the  branchiae ;  note  also  that  the  numerous 
tube  feet  move  as  though  regulated  or  coordinated  by 
some  governing  power,  their  movements  being  thus 
directed  toward  the  attainment  of  some  definite  end 
instead  of  being  at  random.  Examine  a  number  of 
specimens  and  look  for  variations  in  the  size  of  the 
rays.  These  will  show  the  power  of  regenerating  lost 
parts,  which  the  starfish  possesses  to  a  high  degree. 

Structure.  —  Study  the  position  and  arrangement  of 
all  external  parts,  the  spines,  tube  feet,  eyes,  branchiae, 
madreporic  body,  peristome,  the  radial  nerve  in  the 
roof  of  each  groove.  Remove  the  upper  half  of  the 
outer  "  shell  "  and  note  the  internal  organs  :  the  digestive 
system  consisting  of  the  stomach  and  digestive  glands ; 
the  internal  parts  of  the  water-vascular  system,  the  water 
sacs  and  the  "  stone  canal" ;  the  reproductive  glands; 
note  the  radial  plan  of  structure  (Figs.  46,  323,  330). 

Exhibit  a  series  of  eggs,  showing  different  stages 
of  segmentation,  also  the  larval  forms  of  the  starfish. 


PRACTICAL   ZOOLOGY  35 

Sea  Urchin 

Aquarium  Study.  —  Note  its  mode  of  locomotion,  its 
sensitiveness,  its  movements  in  righting  itself  after  hav- 
ing been  turned  bottom  side  up. 

Structure.  —  Trace  all  the  resemblances  you  can  be- 
tween its  external  structure  and  that  of  the  starfish. 

> 

Note  that  in  spite  of  their  difference  in  shape  their 
likeness  in  structure  is  very  marked.  Study  the  diges- 
tive system,  the  teeth,  and  the  long  intestine.  Note  the 
radial  symmetry  (Figs.  48,  226,  237,  293,  294). 

Exhibit  the  larval  stages. 

The  cake  urchin  (Eckinarachnius)  and  the  holothurian 
(Holothuria>  Thyone,  or  Synaptd)  may  be  used  for  com- 
parison (Fig.  49).  No  other  group  of  animals  shows  so 
well  as  the  echinoderms  that  the  same  plan  of  structure 
may  be  associated  with  the  greatest  diversity  of  external 
form. 

Annulata 
Earthworm 

Field  or  Vivarium  Study.  —  Study  living  specimens 
out  of  doors,  note  their  castings  along  paths,  the  amount 
of  earth  brought  up  ;  the  diameter  of  the  burrows  ;  trace 
the  latter  down  into  the  soil.  Place  a  live  worm  on  the 
surface  of  the  soil  and  note  its  mode  of  locomotion ;  its 
method  of  burrowing ;  the  protection  from  enemies 
afforded  by  its  color;  draw  one  out  of  its  burrow  and 
note  the  resistance ;  touch  a  worm  with  a  bristle  and 
note  its  sensitiveness ;  note  the  effect  of  plugging  the 
mouth  of  the  burrow  with  bits  of  straw,  leaves,  etc. 

Watch  the  pulsation  of  the  dorsal  blood  vessel. 

Structure.  —  Note  the  shape  of  the  body,  the  rings 
composing  it,  the  girdle,  the  mouth,  the  anus,  the  open- 
ings of  the  reproductive  glands,  the  bristles. 

Cut  an  alcoholic  or  formalin  specimen  open  along  the 


36       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

middle  of  the  back  and  examine  the  muscular  wall  of 
the  body;  the  tough,  transparent  cuticle;  the  alimen- 
tary tube  within  with  the  blood  vessel  and  digestive 
gland  on  its  upper  side ;  the  partitions  connecting  the 
digestive  tube  with  the  body  wall ;  the  long  series  of 
cavities  nearly  separated  from  one  another  by  these  par- 
titions, the  whole  forming  the  body  cavity ;  the  continu- 
ous digestive  canal  opening  at  each  end  to  the  exterior ; 
the  pair  of  excretory  organs  in  each  separate  cavity. 
Study  the  digestive  tube,  consisting  of  pharynx,  esopha- 
gus, crop,  gizzard,  and  intestine ;  note  the  structure 
of  the  wall  of  the  tube  in  each  of  these  regions ;  the 
supra-esophageal  ganglion  or  "brain"  lying  above  the 
pharynx  ;  the  nerve  cord  below  the  alimentary  canal ; 
the  reproductive  glands  along  the  anterior  part  of  the 
canal ;  note  the  bilateral  arrangement  of  all  organs  ;  also 
that  the  principal  parts  of  the  circulatory  system  lie 
above,  and  of  the  nervous  system  below,  the  digestive 
system  (Fig.  52). 

Draw  attention  to  the  economic  and  geologic  impor- 
tance of  the  earthworm  in  overturning  the  soil  as  it 
feeds  and  constructs  its  burrows.  If  cocoons  (egg  cap- 
sules) can  be  found  (often  attached  to  straws  around 
manure  heaps)  examine  the  various  stages  of  develop- 
ment of  the  earthworm. 

The  leech  and  Nereis  (Fig.  215)  or  Arenicola  (Fig.  274) 
may  be  used  for  comparison. 

Arthropoda 

i.   CRUSTACEA 

Crayfish  or  Lobster 

Material.  —  Live  specimens  of  the  former  may  be 
kept  indefinitely  in  aquarium  jars  containing  algae  and 
supplied  at  intervals  with  a  few  crumbs  or  fragments  of 
beef  or  fish. 


PRACTICAL   ZOOLOGY  37 

Aquarium  Study.  —  Watch  their  movements  when 
walking  and  swimming ;  the  various  motions  of  which 
the  legs  are  capable ;  the  movements  of  the  antennae, 
eyes,  and  swimmerets ;  the  position  of  the  abdomen  ; 
the  manner  in  which  food  (a  scrap  of  fresh  beef)  is  held 
and  pieces  put  into  the  mouth ;  the  movements  of  the 
jaws  ;  of  the  breathing  organs  ;  the  position  of  the  eggs, 
if  a  female  "in  berry"  can  be  obtained;  the  means  of 
offense  and  defense. 

Structure.  —  With  a  dead  specimen,  preferably  alco- 
holic, note  the  hard  covering  of  the  body ;  the  two 
regions  (cephalothorax  and  abdomen);  the  rings  or 
segments  of  which  the  latter  is  composed ;  the  membra- 
nous parts  between  adjacent  rings ;  the  indications  of 
segmentation  seen  on  the  under  side  of  the  cephalo- 
thorax; the  number  and  structure  of  jointed  appendages 
on  the  abdomen ;  the  use  of  each  kind ;  the  number, 
structure,  and  use  of  the  locomotor  appendages  on  the 
cephalothorax;  the  specialization  of  each  pair  for  par- 
ticular functions ;  the  relation  between  legs  and  gills ; 
the  arrangement,  structure,  and  use  of  the  various  mouth 
parts  ;  the  structure  of  the  eyes  and  "  feelers  ' ;  the  ear  ; 
the  protection  of  the  gills  ;  endeavor  to  make  out  the 
fundamental  plan  of  structure  which  underlies  the  great 
diversity  of  form  shown  by  the  various  appendages 

(Fig.  54). 

Cut  through  the  shell  along  each  side  and  remove  the 
upper  part,  thus  exposing  the  internal  organs.  Note 
the  large'  muscles  in  the  abdomen ;  the  pericardium 
and  heart  with  the  large  artery  running  backward  along 
the  middle  line  of  the  large  abdominal  muscle  ;  the 
stomach  with  the  bonelike  parts  in  its  walls ;  the  intes- 
tine ;  the  digestive  glands ;  the  esophagus ;  the  repro- 
ductive glands ;  the  "green  glands"  (in  the  crayfish); 
the  nerve  cord  lying  below  the  digestive  system  ;  the 


38       STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 

"  brain  " ;  note  the  tendency  of  ganglia  to  fuse  into  larger 
nerve  centers  (Figs.  55,  267). 

Exhibit  a  series  of  larval  crayfishes  or  lobsters,  show- 
ing the  changes  undergone  at  each  molting. 

Try  to  get  specimens  of  the  lobster,  showing  the  re- 
generation of  lost  parts,  specially  the  pincers. 

Call  attention  to  resemblances  in  structure  between 
earthworm  and  crayfish  ;  note  in  the  latter  the  tendency 
to  collect  into  definite  regions  the  organs  devoted  to 
definite  uses ;  also  that  every  segment  bears  a  pair  of 
jointed  appendages. 

Use  the  crab  (Callinectes}  for  comparison. 

2.    INSECTA 
Grasshopper 

Living  specimens  may  be  kept  in  boxes  or  jars  cov- 
ered with  gauze  or  netting  and  kept  supplied  with  plenty 
of  fresh  grass  or  wheat. 

Field  and  Vivarium  Study,  —  Note  how  the  insect 
walks,  leaps,  and  flies  ;  the  length  of  a  single  leap  ;  how 
the  food  is  held  and  eaten ;  the  use  of  the  various  sense 
organs,  as  eyes  and  feelers;  the  mode  of  breathing; 
the  protective  coloration  of  the  body. 

Structure.  —  Note  the  three  regions  of  the  body  (head, 
thorax,  and  abdomen),  comparing  with  crayfish  and 
spider ;  study  the  structure  of  each  region ;  the  appen- 
dages borne  by  each  region  ;  their  use  ;  the  structure  of 
each  kind  of  appendage  and  its  adaptation  to  tits  special 
function ;  the  spiracles ;  the  ovipositor  on  the  female ; 
the  ear  (Figs.  64,  219,  276,  295,  344,  352). 

Cut  open  a  specimen  lengthwise  and  note  the  parts 
of  the  digestive  system ;  the  muscles ;  the  reproductive 
organs ;  the  structure  and  arrangement  of  the  nervous 
system.  Examine  tracheal  tubes  with  the  microscope 
(Figs.  239,  277,  278). 


PRACTICAL  ZOOLOGY  39 

Compare  the  plan  of  structure  of  the  grasshopper 
with  that  of  the  earthworm,  crayfish,  and  spider. 

Try  to  get  young  grasshoppers  and  note  the  changes 
which  are  shown  at  the  successive  molts. 

Butterfly 

Field  and  Vivarium  Study.  —  Note  its  wavering  flight, 
the  way  it  walks,  the  position  of  the  wings  when  at 
rest,  the  use  of  the  proboscis  when  gathering  nectar, 
the  species  of  plants  visited,  the  position  of  the  pro- 
boscis when  not  in  use. 

Structure.  —  Note  the  similarity  of  structure  to  the 
grasshopper ;  the  differences  between  the  wings  of  the 
insects  ;  the  greater  uniformity  in  the  structure  of  the  legs 
of  the  butterfly ;  the  position  of  the  eyes ;  the  shape 
of  the  antennae ;  the  structure  of  the  proboscis ;  the 
microscopic  appearance  of  the  scales  on  the  surface  of 
the  wings  (Figs.  70,  71,  221,  238,  241). 

Examine  the  larva,  noting  its  shape  and  color ;  its  mode 
of  locomotion ;  locomotor  organs ;  food  and  method  of 
feeding  (Fig.  73).  Put  mature  larvae  (of  Mourning  Cloak 
butterfly,  for  example)  in  a  glass-covered  box  and  watch 
them  as  they  change  to  the  pupa  stage. 

Study  the  chrysalis  and  eggs,  if  obtainable  (Fig.  368). 

The  bee,  fly,  and  beetle  may  be  used  for  comparison. 

3.    ARACHNIDA 
Spider 

Field  and  Vivarium  Study.  —  Study  the  mode  of  loco- 
motion; the  position,  arrangement,  and  captured  con- 
tents of  a  web ;  the  position  of  the  spider  in  the  web. 
Put  a  spider  into  a  large  pasteboard  or  wooden  box, 
cover  with  a  sheet  of  glass,  and  note  how  the  silk  is 
spun  and  the  web  is  constructed.  Look  for  cocoons 
and  study  their  structures  and  contents. 


40       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

Structure.  —  Note  the  same  regions  as  in  the  crayfish 
(cephalothorax  and  abdomen) ;  study  the  points  of  re- 
semblance and  of  difference;  indications  of  segmenta- 
tion shown  by  each  ;  compare  the  number,  position,  and 
structure  of  the  legs ;  study  the  spinnerets,  their  posi- 
tion, number,  and  form ;  the  mouth  parts  ;  the  eyes ; 
the  structure  and  appendages  of  the  skin  ;  the  openings 
of  the  breathing  chambers  (Figs.  81,  82,  216,  223). 

Dissect  alcoholic  specimens  in  a  dish  of  weak  alcohol 
or  of  water,  and  note  the  arrangement  of  the  digestive 
and  nervous  systems. 

Mollusca 

Mussel 

The  river  mussels  may  be  kept  in  aquariums  having 
two  to  four  inches  of  sand  or  mud  on  the  bottom. 

Aquarium  Study.  —  Study  the  movements  of  the  ani- 
mal ;  the  opening  and  closing  of  the  shell ;  the  position 
and  use  of  the  foot ;  of  the  siphons ;  the  sensitiveness 
of  the  siphonal  tentacles ;  the  incurrent  and  excurrent 
streams  of  water  (Fig.  86). 

Structure.  —  Examine  a  shell,  noting  the  two  similar 
valves ;  the  hinge  and  hinge  ligament ;  the  hinge  teeth 
(if  present);  the  "epidermis,"  lines  of  growth,  and 
nacre ;  the  scars  left  by  the  muscles  (Fig.  296). 

Examine  the  soft  part  of  the  body,  the  mantle  lobes ; 
the  gills;  the  body  and  the  foot;  the  palpi  and  the 
mouth ;  the  anal  opening ;  the  adductor,  protractor,  and 
retractor  muscles. 

Cut  through  the  body  lengthwise  and  trace  the  course 
of  the  alimentary  canal.  Endeavor  to  trace  the  parts 
of  the  nervous  system ;  the  cerebral  and  the  visceral 
ganglia;  the  heart;  the  digestive  gland  (Figs.  244,  332). 
Make  three  or  four  cross  sections  from  a  specimen 


PRACTICAL  ZOOLOGY  41 

hardened  in  formalin  or  alcohol  and  study  the  supra- 
branchial  canal,  gills,  etc.  (Fig.  275). 
Examine  the  gills  for  eggs  and  young. 

Land  or  Water  Snail 

Field  or  Aquarium  Study. — -Study  its  movements  ;  its 
mode  of  respiration;  its  feeding;  the  movements  of  the 
"  rasp  " ;  the  use  of  the  "  feelers  "  ;  the  manner  in  which 
the  body  is  protruded  from  the  shell  and  retracted. 

Structure.  —  Compare  its  shell  with  that  of  the 
mussel.  Note  the  whorls ;  the  lines  of  growth ;  the 
attachment  of  the  body  to  the  shell  (Fig.  297). 

Remove  the  soft  parts  and  study  their  structure, 
the  digestive  system  (Fig.  227);  the  large  liver;  the 
heart,  the  brain,  and  nervous  system  (Figs.  243,  331, 
351).  Note  that  the  bodies  of  both  these  mollusks 
are  unsegmented  and  are  without  appendages. 

Snails  kept  in  aquariums  frequently  attach  their  eggs 
to  the  sides  of  the  jar  or  to  water  plants.  The  seg- 
mentation of  the  egg  and  the  development  of  the  larva 
may  be  studied  with  a  low  power  or  with  a  hand  lens. 

VERTEBRATES 

Vertebrata 

Fish 

Aquarium  Study.  —  Study  living  specimens  in  the 
aquarium,  their  movements  of  locomotion  and  of  the 
various  fins  ;  mouth,  gill  covers,  and  gills  ;  the  eyes  ; 
the  method  of  feeding  and  of  respiration  ;  the  distribu- 
tion of  colored  spots  on  the  body ;  the  adaptation  of 
shape  to  locomotion  in  the  water ;  test  the  use  of  each 
fin  by  binding  them  separately  to  the  body  by  means  of 
rubber  bands  slipped  on  over  the  fish's  head. 

Structure.  —  On  a  dead  specimen  note  the  bilateral 


42       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

symmetry ;  the  head ;  the  absence  of  a  neck ;  the  body 
and  scales ;  the  position  and  structure  of  the  fins ;  the 
shape  and  structure  of  the  mouth ;  the  teeth ;  nostrils ; 
tongue  ;  gills  ;  the  lateral  line  ;  microscopic  structure  of 
scales  and  gills  (Figs.  119,  320). 

Dissect  away  the  skin '  from  one  side  and  study  the 
arrangement  of  the  muscles. 

Cut  open  a  specimen  and  study  the  position  and 
relation  of  the  internal  organs ;  the  peritoneum  ;  the 
digestive  (Fig.  246),  circulatory,  and  reproductive  systems 
(Figs.  268,  269,  272);  the  structure  of  the  heart;  open 
the  skull  and  examine  the  brain  and  the  principal  nerves 
arising  from  it  (Fig.  336).  Remove,  by  boiling,  the  flesh 
from  the  skeleton,  and  study  the  structure  of  the  latter 
(Fig.  309). 

Make  a  microscopic  examination  of  the  blood 
(Fig.  262). 

Obtain  (from  one  of  the  state  hatcheries,  if  necessary) 
a  series  of  living  eggs  and  embryos,  and  study  the  de- 
velopment of  the  fish. 

If  opportunity  offers,  a  fish  market  may  be  visited 
and  an  examination  made  of  the  various  kinds  of  food 
fishes. 

Frog 

Vivarium  and  Aquarium  Study.  —  Keep  live  speci- 
mens in  aquarium  jars  or  in  boxes  containing  damp 
moss.  Study  the  manner  in  which  the  frog  creeps, 
leaps,  swims,  breathes,  moves,  and  closes  its  eyes, 
catches  flies ;  the  position  of  the  body  at  rest ;  with  a 
thermometer  try  to  get  the  natural  temperature  of  the 
body. 

Structure.  —  In  a  recently  killed  specimen  note  the 
color  and  structure  of  the  skin,  the  position  of  eyes, 
ears,  nostrils,  lips,  the  position  and  arrangement  of  the 
lips  and  the  teeth,  the  shape  and  mode  of  attachment  of 


PRACTICAL   ZOOLOGY  '  43 

the  tongue ;  the  sticky  saliva  and  its  use ;  the  absence 
of  a  neck. 

Dissect  away  the  skin  and  study  the  shape  and 
attachments  of  the  underlying  muscles.  Open  the 
abdomen  and  study  the  arrangement  of  the  internal 
organs,  the  digestive,  circulatory,  respiratory,  excretory, 
and  reproductive  systems  (Figs.  273,  282);  the  structure 
of  the  heart.  Open  the  skull  and  examine  the  brain 
(Fig.  337) ;  trace  the  course  of  the  principal  nerves. 
Study  the  principal  parts  of  the  skeletal  system  and 
compare  with  that  of  the  fish  (Fig.  284).  Examine 
the  circulation  of  the  blood  as  seen  in  the  web  of  the 
foot  (Fig.  263).  Study  the  corpuscles  in  a  drop  of  fresh 
blood  (Figs.  260,  261). 

Collect  the  eggs  of  frogs  or  toads  in  the  spring,  keep 
them  in  an  aquarium,  and  watch  the  development  of  the 
tadpole.  Have  a  series  of  tadpoles  showing  the  gradual 
metamorphosis  into  the  adult  stage. 

Draw  attention  to  the  changes  of  structural  adapta- 
tion necessitated  by  the  change  from  the  aquatic  to  the 
aerial  mode  of  life. 

Turtle 

Water  or  land  turtles  may  be  used,  and  may  be  kept 
alive  indefinitely  in  a  damp  box  in  the  laboratory. 

Field,  Vivarium,  and  Aquarium  Study.  —  On  some  of 
the  field  excursions  look  for  turtles  in  their  native  haunts, 
and  learn  as  much  as  possible  of  their  habits.  In  the 
laboratory  note  how  the  turtle  walks,  its  clumsy  motions, 
rate  of  speed ;  the  motions  of  and  positions  taken  by  its 
head,  legs,  tail ;  movements  of  the  eyelids,  nostrils ;  the 
respiratory  movements.  Put  the  turtle  into  water  and 
watch  its  movements  when  swimming  and  diving. 

Structure.  —  Study  the  external  covering,  its  structure, 
color,  and  modifications  on  the  body,  head,  legs,  and  tail ; 


44       STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 

compare  with  the  fish  and  the  frog ;  the  head,  its  shape 
and  various  parts  composing  it,  the  jaws,  eyes,  nostrils ; 
note  the  absence  of  teeth ;  the  tongue. 

Remove  the  lower  half  of  the  shell  and  study  the 
internal  organs  composing  the  digestive,  circulatory, 
reproductive,  and  excretory  systems;  compare  the 
structure  of  the  heart  with  that  of  the  fish  and  the 
frog  (Fig.  273). 

On  a  skeleton  note  the  various  parts  which  are 
attached  to  the  shell ;  the  skull  and  neck ;  the  hyoid 
apparatus,  the  structure  of  the  limbs  and  tail ;  compare 
the  hyoid  apparatus  and  the  ribs  with  those  of  the  frog 
(Fig.  312). 

If  eggs  can  be  obtained,  note  the  shape,  structure  of 
the  shell,  and  the  stages  of  development  of  the  young. 

Bird 

Sparrows  or  pigeons  may  be  used. 

Field  Study.  —  Note  its  general  mode  of  life,  whether 
solitary  or  gregarious ;  relations  to  other  birds  and  to 
man ;  its  manner  of  flight  and  of  walking ;  feeding 
habits ;  size,  shape,  and  coloration  of  the  body ;  varia- 
tions in  coloration  at  different  seasons  of  the  year; 
position  and  structure  of  the  nest,  number,  shape,  size, 
and  color  of  eggs,  number  of  broods  each  year,  season 
when  broods  are  produced,  and  number  of  young  in 
each  brood ;  enemies ;  song ;  if  a  living  specimen  can 
be  obtained,  test  the  body  temperature  with  a  ther- 
mometer. 

Structure.  —  With  a  recently  killed  specimen,  study 
the  shape  of  the  body,  the  direction  of  its  axis ;  the 
position  and  mobility  of  the  head,  wings,  legs,  and  tail ; 
the  distribution  of  the  various  feathers,  their  structure 
(Figs.  139,  302).  Remove  the  latter  and  note  the  feather 
tracts  and  the  skin.  Study  the  shape  and  structure  of 


PRACTICAL  ZOOLOGY  45 

the  head,  the  beak,  eyes,  nostrils,  and  ears;  compare 
with  the  turtle.  Examine  the  wings  and  legs,  noting 
the  direction  and  movements  of  the  various  segments ; 
the  structure  and  movements  of  the  parts  of  the  foot ;  the 
position  of  the  principal  muscles,  their  uses. 

Open  the  body  and  study  the  digestive,  circulatory, 
respiratory,  and  reproductive  systems  (Figs.  248,  273); 
the  air  spaces  among  the  muscles ;  the  structure  of  the 
heart  and  brain  as  compared  with  the  vertebrates  pre- 
viously studied  (Fig.  338);  the  microscopic  appearance 
of  the  blood  corpuscles  (Fig.  262). 

Prepare  or  purchase  a  skeleton  and  study  the  arrange- 
ment of  its  various  parts  and  the  structure  of  the  dif- 
ferent bones,  comparing  with  the  fish,  frog,  and  turtle 
(Fig.  3i3> 

Study  the  structure  of  the  egg  (Fig.  358),  and  the 
development  of  the  young  (a  convenient  and  satisfac- 
tory substitute  is  the  hen's  egg)  (Figs.  365,  366). 

Draw  attention  to  the  economic  value  of  the  bird 
studied. 

Mammal 

The  cat  or  rabbit  may  be  used. 

Laboratory  Study.  —  Study  the  motions  of  the  animal  as 
it  walks,  runs,  leaps,  its  position  when  at  rest ;  food  and 
mode  of  feeding ;  respiratory  movements ;  motions  of 
head,  legs,  tail,  ears,  eyes ;  mode  of  cleaning  its  fur ; 
body  temperature ;  protective  coloration. 

Structure.  —  On  a  recently  killed  specimen  note  the 
general  shape  of  the  body  and  direction  of  its  axis; 
the  position  and  mode  of  attachment  of  the  append- 
ages ;  the  hairy  covering,  the  groups  of  specialized 
hairs  in  certain  positions,  the  microscopic  appearance 
of  hair.;  the  mobility  of  the  skin ;  its  firmer  attachment 
in  certain  places,  compare  with  the  external  covering  of 
fish,  frog,  turtle,  and  bird.  Study  the  shape  and  structure 


46       STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

of  the  head,  the  position  and  structure  of  ears,  eyes,  nos- 
trils, and  mouth. 

Remove  the  skin  from  the  body  and  study  the  position 
and  attachment  of  the  more  important  muscles,  their  uses. 
Open  the  body  and  examine  the  organs  composing  the 
digestive,  circulatory,  respiratory  (Fig.  283),  excretory, 
and  reproductive  systems  (Fig.  250).  Note  the  posi- 
tion and  structure  of  the  teeth  and  their  fitness  to  mas- 
ticate the  special  kind  of  food  the  animal  eats ;  the 
surface  of  the  tongue  and  its  adaptation  as  an  organ 
for  cleaning  the  fur;  compare  the  heart  and  brain 
(Fig.  339),  and  the  microscopic  appearance  of  the  blood 
corpuscles  with  those  of  other  vertebrates  examined 
(Fig.  259). 

Trace  the  course  of  some  of  the  principal  blood  vessels 
and  nerves. 

Examine  a  skeleton  and  compare  with  that  of  the 
other  vertebrates  studied  (Fig.  303). 

A  series  of  preparations  of  fetal  kittens  or  rabbits  may 
be  examined. 


CHAPTER    II 

THE   CLASSIFICATION   OF  ANIMALS 

THE  Kingdom  of  Nature  is  a  literal  Kingdom.  Order 
and  beauty,  law  and  dependence,  are  seen  everywhere. 
Amidst  the  great  diversity  of  the  forms  of  life,  there  is 
unity ;  and  this  suggests  that  there  is  one  general  plan, 
but  carried  out  in  a  variety  of  ways. 

Naturalists  have  ceased  to  believe  that  each  animal 
or  group  is  a  distinct,  circumscribed  idea.  "  Every  ani- 
mal has  a  something  in  common  with  all  its  fellows : 
much  with  many  of  them ;  more  with  a  few ;  and, 
usually,  so  much  with  several,  that  it  differs  but  little 
from  them."  The  object  of  classification  is  to  bring 
together  the  like,  and  to  separate  the  unlike.  But  how 
shall  this  be  done  ?  To  a-rrange  a  library  in  alphabeti- 
cal order,  or  according  to  size,  binding,  date,  or  lan- 
guage, would  be  unsatisfactory.  We  must  be  guided  by 
some  essential  character.  We  must  decide  whether  a 
book  is  poetry  or  prose;  if  poetry,  whether  dramatic, 
epic,  lyric,  or  satiric ;  if  prose,  whether  history,  philoso- 
phy, theology,  philology,  science,  fiction,  or  essay.  The 
more  we  subdivide?  these  groups,  the  more  difficult  the 
analysis. 

A  classification  of  animals,  founded  on  external  re- 
semblances —  as  size,  color,  or  adaptation  to  similar 
habits  of  life  —  would  be  worthless.  It  would  bring 
together  fishes  and  whales,  birds  and  bats,  worms  and 
eels.  Nor  should  it  be  based  on  any  one  character,  as 
the  quality  of  the  blood,  structure  of  the  heart,  develop- 
ment of  the  brain,  embryo  life,  etc. ;  for  no  character  is 

47 


48       STRUCTURAL   AND   SYSTEMATIC  ZOOLOGY 

of  the  same  value  in  every  tribe.  A  natural  classification 
must  rest  on  those  prevailing  characters  which  are  the 
most  constant.^  And  such  a  classification  can  not  be 
linear.  It  is  impossible  to  arrange  all  animal  forms 
from  the  sponge  to  man  in  a  single  line,  like  the  steps 
of  a  ladder,  according  to  rank.  Nature  passes  in  so 
many  ways  from  one  type  to  another,  and  so  multiplied 
are  the  relations  between  animals,  that  one  series  is  out 
of  the  question.  There  is  a  number  of  series,  and 
series  within  series,  sometimes  proceeding  in  parallel 
lines,  but  more  often  divergent.  The  animals  arrange 
themselves  in  radiating  groups,  each  group  being  con- 
nected, not  with  two  groups  merely,  one  above  and  the 
other,  below,  but  with  several.  Life  has  been  likened  to 
a  great  tree  with  countless  branches  spreading  widely 
from  a  common  trunk,  and  deriving  their  origin  from  a 
common  root ;  branches  bearing  all  manner  of  flowers, 
every  fashion  of  leaves,  and  all  kinds  of  fruit,  and  these 
for  every  use. 

The  groups  into  which  we  are  able  to  cast  the  various 
forms  of  animal  development  are  very  unequal  and  dis- 
similar. We  must  remember  that  a  genus,  order,  or 
class  is  not  of  the  same  value  throughout  the  kingdom. 
Moreover,  each  division  is  allied  to  others  in  different 
degrees  —  the  distance  between  any  two  being  the 
measure  of  that  affinity.  The  lines  between  some  are 
sharp  and  clear,  between  others  indefinite.  Like  the 
islands  of  an  archipelago,  some  groups  merge  into  one 
another  through  connecting  reefs,  others  are  sharply 
separated  by  unfathomable  seas,  yet  all  have  one  com- 
mon basis.  Links  have  been  found  revealing  a  relation- 
ship, near  or  distant,  even  between  animals  whose  forms 
are  very  unlike.  There  are  fishes  {Dipnoi)  with  some 
amphibian  characters,  and  fishlike  amphibians  (Ajcolotl). 
The  extinct  ichthyosaurus  was  a  lizard  with  fish  charac- 


THE   CLASSIFICATION   OF   ANIMALS  49 

teristics.  Birds  seem  isolated,  but  they  are  closely  con- 
nected with  reptiles  by  fossil  forms.  Even  the  great 
gap  in  the  animal  kingdom  —  that  separating  verte- 
brates and  invertebrates  —  is  partially  bridged  on  the 
one  side  by  amphioxus,  and  on  the  other  by  balano- 
glossus  (a  wormlike  animal)  and  the  tunicates. 

We  have,  then,  groups  subordinate  to  groups,  and 
interlocking,  but  not  representing  so  many  successive 
degrees  of  organization.  For,  as  already  intimated, 
complication  of  structure  does  not  rise  in  continuous 
gradation  from  one  group  to  another.  Every  type  starts 
at  a  lower  point  than  that  at  which  the  preceding  class 
closes  ;  so  that  the  lines  overlap.  While  one  class,  as  a 
whole,  is  higher  than  another,  some  members  of  the 
higher  class  may  be  inferior  to  some  members  of  the 
lower  one.  Thus,  certain  starfishes  are  structurally 
more  complex  than  certain  mollusks ;  and  the  nautilus 
is  above  the  worm.  The  groups  coalesce  by  their  in- 
ferior or  less  specialized  members;  e.g.,  the  fishes  do  not 
graduate  into  amphibians  through  their  highest  forms, 
but  the  two  come  closest  together  low  down  in  the 
scale.  Among  the  craniate  animals  the  lines  of  descent 
of  the  various  classes  may  be  represented  as  diverging 
and  ascending  from  a  point  occupied  by  a  fishlike 
ancestor. 

A  number  of  animals  may,  therefore,  have  the  same 
grade  of  development,  but  conform  to  entirely  different 
types.  While  a  fundamental  unity  underlies  the  whole 
animal  kingdom,  suggesting  a  common  starting  point, 
we  recognize  several  distinct  plans  of  structure.5  Ani- 
mals like  the  amoeba,  with  no  cellular  tissues  and  no 
true  eggs,  form  the  branch  Protozoa.  Animals  like  the 
sponge,  with  independent  cells,  one  excurrent  and  many 
incurrent  openings,  form  the  branch  Porifera.  Animals 
like  the  coral,  unlike  all  others,  have  an  alimentary  canal 
DODGE'S  GEN.  ZOOL. —  4 


50       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

but  no  body  cavity,  have  no  separate  nervous  and  vascu- 
lar regions,  and  the  parts  of  the  body  radiate  from  a 
center.  Such  form  a  branch  called  Ccelenterata.  Ani- 
mals like  the  starfish,  having  also  a  radiating  body,  but 
a  closed  alimentary  canal,  and  a  distinct  symmetrical 
nervous  system,  constitute  the  branch  Echinodermata* 
Animals  like  the  angleworm,  bilaterally  symmetrical, 
one-jointed,  or  composed  of  joints  following  each  other 
from  front  to  rear,  with  no  jointed  limbs,  constitute  the 
branch  Annulata.  Animals  like  the  snail,  with  a  soft, 
unjointed  body,  a  mantle,  a  foot,  a  two  or  three  cham- 
bered heart,  and  a  nervous  system  in  the  form  of  a  ring 
around  the  gullet,  constitute  the  branch  Mollusca.  Ani- 
mals like  the  bee,  with  a  jointed  body  and  jointed  limbs, 
form  the  branch  Arthropoda.  Anifnals  like  the  ox,  hav- 
ing a  double  nervous  system,  one  (the  sympathetic) 
lying  on  the  upper  side  of  the  alimentary  canal,  the 
other  and  main  part  (spinal)  lying  along  the  back,  and 
completely  shut  off  from  the  other  organs  by  a  partition 
of  bone  or  gristle,  known  as  the  "vertebral  column,"  and 
having  limbs,  never  more  than  four,  always  on  the  side 
opposite  the  great  nervous  cord,  constitute  the  branch 
Vertebrata. 

Comparing  these  great  divisions,  we  see  that  the  ver- 
tebrates differ  from  all  the  others  chiefly  in  having  a 
double  body  cavity  and  a  double  nervous  system,  the 
latter  lying  above  the  alimentary  canal ;  while  inverte- 
brates have  one  cavity  and  one  nervous  system,  the 
latter  being  placed  mainly  below  the  alimentary  canal. 

But  there  are  types  within  types.  Thus,  there  are 
five  modifications  of  the  vertebrate  type  —  fish,  amphib- 
ian, reptile,  bird,  and  mammal ;  and  these  are  again 
divided  and  subdivided,  for  mammals,  e.g.,  differ  among 
themselves.  So  that  in  the  end  we  have  a  constellation 
of  groups  within  groups,  founded  on  peculiar  characters 


THE   CLASSIFICATION    OF   ANIMALS  51 

of  less  and  less  importance,  as  we  descend  from  the 
general  to  the  special. 

Individuals  are  the  units  of  the  Animal  Creation. 
An  animal  existence,  complete  in  all  its  parts,  is  an 
individual,  whether  separate,  as  man,  or  living  in  a  com- 
munity, as  the  coral.7 

Species  is  the  smallest  group  of  individuals  which  can 
be  defined  by  distinct  characteristics,  and  which  is 
separated  by  a  gap  from  all  other  like  groups.  A  well- 
marked  subdivision  of  a  species  is  called  a  variety. 
Crosses  between  species  are  called  hybrids,  as  the 
mule. 

Genus  is  a  group  of  species  having  the  same  essential 
structure.  Thus,  the  closely  allied  species  cat,  tiger, 
and  lion  belong  to  one  genus. 

Family,  or  Tribe,  is  a  group  of  genera  having  a  simi- 
lar form.  Thus,  the  dogs  and  foxes  belong  to  different 
genera,  but  betray  a  family  likeness. 

Order  is  a  group  of  families,  or  genera,  related  to  one 
another  by  a  common  structure.  Cats,' dogs,  hyenas, 
and  bears  are  linked  together  by  important  anatomical 
features ;  their  teeth,  stomachs,  and  claws  show  carniv- 
orous habits. 

Class  is  a  still  larger  group,  comprising  all  animals 
which  agree  simply  in  a  special  modification  of  the  type 
to  which  they  belong.  Thus,  fishes,  amphibians,  rep- 
tiles, birds,  and  mammals  are  so  many  aspects  of  the 
vertebrate  type. 

Branch  is  a  primary  division  of  the  animal  kingdom, 
which  includes  all  animals  formed  upon  one  of  the 
various  types  of  structure ;  as  vertebrate. 

The  branches  are  grouped  into  two  great  Series  (Pro- 
tozoa and  Metazoa),  according  to  their  histological 
structure  and  mode  of  development.8 

These    terms   were    invented    by    Linnaeus,   except 


52       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

Family,  Branch,  and  Series.  To  Linnaeus  we  are  also 
indebted  for  a  scientific  method  of  naming  animals. 
Thus,  the  dog,  in  Zoology,  is  called  Cam's  familiaris, 
which  is  the  union  of  a  generic  and  a  specific  name, 
corresponding  to  the  surname  and  the  Christian  name 
in  George  Washington,  only  the  specific  name  comes 
last.  It  will  be  understood  that  these  are  abstract 
terms,  expressing  simply  the  relations  of  resemblance ; 
there  is  no  such  thing  as  genus  or  species. 

Classification  is  a  process  of  comparison.  He  is  the 
best  naturalist  who  most  readily  and  correctly  recog- 
nizes likeness  founded  on  structural  characters.  As 
it  is  easier  to  detect  differences  than  resemblances,  it 
is  much  easier  to  distinguish  the  class  to  which  an  ani- 
mal belongs  than  the  genus,  and  the  genus  than  the 
species.  In  passing  from  species  to  classes,  the  charac- 
ters of  agreement  become  fewer  and  fewer,  while  the 
distinctions  are  more  and  more  manifest;  so  that  ani- 
mals of  the  same  class  are  more  like  than  unlike,  while 
members  of  distinct  classes  are  more  unlike  than  like. 

To  illustrate  the  method  of  zoological  analysis  by 
searching  for  affinities  and  differences,  we  will  take 
an  example  suggested  by  Professor  Agassiz.  Suppose 
we  see  together  a  dog,  a  cat,  a  bear,  a  horse,  a  cow,  and 
a  deer.  The  first  feature  which  strikes  us  as  common 
to  any  two  of  them  is  the  horn  in  the  cow  and  the  deer. 
But  how  shall  we  associate  either  of  the  others  with 
these  ?  We  examine  the  teeth,  and  find  those  of  the 
dog,  the  cat,  and  the  bear  sharp  and  cutting ;  while 
those  of  the  cow,  the  deer,  and  the  horse  have  flat  sur- 
faces, adapted  to  grinding  and  chewing,  rather  than  to 
cutting  and  tearing.  We  compare  these  features  of 
their  structure  with  the  habits  of  these  animals,  and 
find  that  the  first  are  carnivorous  —  that  they  seize  and 
tear  their  prey;  while  the  others  are  herbivorous,  or 


THE   CLASSIFICATION   OF   ANIMALS  53 

grazing,  animals,  living  only  on  vegetable  substances, 
which  they  chew  and  grind.  We  compare,  further,  the 
horse  and  cow,  and  find  that  the  horse  has  front  teeth 
both  in  the  upper  and  the  lower  jaw,  while  the  cow  has 
them  only  in  the  lower ;  and  going  still  farther,  and 
comparing  the  internal  with  the  external  features,  we 
find  this  arrangement  of  the  teeth  in  direct  relation  to 
the  different  structure  of  the  stomach  in  the  two  ani- 
mals—  the  cow  having  a  stomach  with  four  pouches, 
while  the  horse  has  a  simple  stomach.  Comparing  the 
cow  and  deer,  we  find  the  digestive  apparatus  the  same 
in  both ;  but  though  both  have  horns,  those  of  the  cow 
are  hollow,  and  last  through  life ;  while  those  of  the 
deer  are  solid,  and  are  shed  every  year.  Looking  at 
the  feet,  we  see  that  the  herbivorous  animals  are  hoofed ; 
the  carnivorous,  clawed.  The  cow  and  deer  have  cloven 
feet,  and  are  ruminants;  the  horse  has  a  single  hoof, 
and  does  not  chew  the  cud.  The  dog  and  cat  walk  on 
the  tips  of  their  fingers  and  toes  (digitigrade) ;  the  bear 
treads  on  the  palms  and  soles  (plantigrade).  The  claws  of 
the  cat  are  retractile  ;  those  of  the  dog  and  bear  are  fixed. 
In  this  way  we  determine  the  exact  place  of  each  ani- 
mal. The  dog  belongs  to  the  kingdom  Animalia,  branch 
Vertebrata,  class  Mammalia,  order  Carnivora,  family  Ca- 
nida,  genus  Cants,  species  familiaris,  variety  hound  (it 
may  be),  and  its  individual  name,  perhaps,  is  "  Rover." 
The  cat  differs  in  belonging  to  the  family  Felida,  genus 
Felis,  species  domestica.  The  bear  belongs  to  the  family 
Ursidce,  genus  Ursus,  and  species  ~horribilis,  if  the  grizzly 
is  meant.  The  horse,  cow,  and  hog  belong  to  the  order 
Ungulata ;  but  the  horse  is  of  the  family  Equidcz,  genus 
Equns,  species  caballus ;  the  cow  is  of  the  family  Bovi- 
da,  genus  Bos,  species  tanrus ;  the  pig  is  of  the  family 
Suidcz,  genus  Sns,  species  scrofa,  if  the  domestic  pig  is 
meant. 


54       STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

The  diagram  on  the  opposite  page  roughly  represents 
(for  the  relations  of  animals  can  not  be  expressed  on  a 
plane  surface)  the  relative  positions  of  the  branches  and 
classes  according  to  affinity  and  rank.* 

SERIES    I.  — PROTOZOA 

Animals  whose  bodies  consist  of  a  single  cell,  the 
process  of  reproduction  being  by  division  or  by  budding, 
but  never  by  means  of  true  eggs. 

Branch  I.  —  PROTOZOA 

In  structure  the  Protozoa  are  the  simplest  of  animals, 
consisting  of  only  a  single  cell.  They  are  microscopic 
in  size  and  aquatic  in  habit,  though  in  certain  stages 
of  their  lives  (encystation)  many  of  them  may  endure 
dryness  for  weeks  or  months.  Their  bodies  consist 
mainly  or  wholly  of  protoplasm,  which  may  or  may  not 
be  covered  by  a  cuticle  or  by  a  shell-like  excretion  of 
lime,  chitin,  or  flint,  or  inclose  spicules  of  the  latter 
substance.  The  various  individuals  may  live  separately 
as  single,  independent  organisms,  or  they  may  be  or- 
ganically joined  together  in  clusters  called  colonies. 
They  exhibit  all  the  essential  functions  of  life  —  nutri- 
tion, growth,  nervous  properties,  and  reproduction. 
They  feed  upon  minute  algae,  bacteria,  vegetable  de- 
bris, and  upon  other  microscopic  animals.  Some  forms 
are  parasitic.  It  has  been  shown  by  experiment  that 
many  species  are  sensitive  to  changes  in  the  amount  of 
illumination  to  which  their  bodies  are  exposed  and  to 
various  colors  of  light;  that  they  are  attracted  or  re- 

*  The  student  should  master  the  distinctions  between  the  great  groups, 
or  classes,  before  proceeding  to  a  minuter  classification.  "The  essential 
matter,  in  the  first  place,"  says  Huxley,  "  is  to  be  quite  clear  about  the 
different  classes,  and  to  have  a  distinct  knowledge  of  all  the  sharply  de- 
finable modifications  of  animal  structure  which  are  discernible  in  the  Ani- 
mal Kingdom." 


THE   CLASSIFICATION    OF   ANIMALS 


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56       STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 

pelled  by  the  presence  of  different  chemical  substances 
in  the  water ;  and  that  they  are  sensitive  to  contact  with 
foreign  bodies  and  with  one  another.  Thus  it  is  proved 
that  these  simple  organisms  possess  the  rudiments  of 
the  nervous  .properties  seen  in  the  higher  animals.  Cer- 
tain species  contain  a  green  coloring  substance  (haema- 
tochrome)  which  is  chemically  allied  to  the  chlorophyll 
of  plants.  Others,  again,  pass  through  amoeboid  stages 
resembling  similar  phases  in  the  development  of  some 
of  the  lowest  plants.  Because  of  these  resemblances 
some  of  the  Protozoa  are  almost  indistinguishable  from 
the  lowest  members  of  the  plant  kingdom  (Protophyta). 
On  account  of  the  apparent  simplicity  of  their  struc- 
ture, it  is  difficult  to  select  features  by  means  of  which 
the  animals  in  this  group  may  be  classified.  The  diffi- 
culty is  further  increased  by  the  fact  that  in  the  course 
of  their  development  some  forms  pass  through  stages  in 
which  they  resemble  other  species  in  the  same  branch. 
In  every  case,  however,  it  is  found  that  certain  phases 
of  their  development  predominate,  and  these  well- 
marked  phases  permit  of  dividing  the  Protozoa  into 
five  classes. 

CLASS   i .  —  Rhizopoda 

These  are  Protozoa  which  are  predominantly  amoe- 
boid in  shape  and  which  move  by  means  of  pseudopodia, 
as  the  slow-moving  protrusions  of  the  protoplasmic  body 
substance  are  called  (Figs,  i,  213).  The  body  usually 
contains  a  nucleus  and  a  contractile  vacuole.  The  com- 
mon amoeba  or  proteus  animalcule  belongs  in  this  class 
(Fig.  i).  Some  of  the  Rhizopods  secrete  shells  of 
chitin  (Arcella),  or  construct  a  covering  made  of  parti- 
cles of  sand  (Diff.ugia).  Both  of  these  organisms  are 
found  in  fresh  water  in  America.  The  most  primitive 
representative  of  the  group  is  Protamceba,  in  which 


PROTOZOA 


57 


neither  nucleus  nor  contractile  vacuole  has  been  dis- 
covered. Pelomyxa,  a  fresh-water  form,  may  reach  the 
size  of  eight  millimeters  (.3  of  an  inch)  in  diameter. 

An  amoeba  is  a  naked  fresh-water  Rhizopod,  contain- 
ing a  nucleus  and  a  contractile  vacuole,  the  body  sub- 


FIG.  i.  —  Amoeba,  showing  the  structure  of  the  body  and  the  changes  which  take  place 
during  division.  The  dark  body  in  each  figure  is  the  nucleus  ;  the  transparent  circle, 
the  contractile  vacuole  ;  the  protrusions  of  the  body  substance,  pseudopodia  ;  the 
outer,  clear  portion  of  the  body,  the  ectosarc  ;  the  granular  portion,  the  endosarc  ; 
the  granular  masses,  food  vacuoles.  Much  magnified. 

stance  consisting  of  two  rather  distinct  layers,  the  outer 
being  quite  clear  and  transparent,  while  the  inner  is  usu- 
ally filled  with  granules  and  ingested  particles.  During 
movement  the  shape  of  the  body  is  constantly  changing, 
owing  to  the  protrusion  and  withdrawal  of  the  pseudo- 


58       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

podia.     Food  is  taken  into  the  body  at  any  point,  there 
being  no  mouth. 

A  Foraminifer  differs  from  an  amoeba  in  having  an 
apparently  simpler  body,  the  protoplasm  being  without 
layers  or  cavity ;  its  pseudopodia  are  long  and  threadlike, 
and  may  unite  where  they  touch  each  other.  It  has  the 
property  of  secreting  an  envelope,  usually  of  carbonate 
of  lime.  The  shell  thus  formed  is  sometimes  of  extraor- 
dinary complexity  and  singular  beauty.  In  addition  to 
the  terminal  aperture,  it  is  generally  perforated  by 


a 


FIG.  2. —  Rhizopods  :  a,  shell  of  a  monothalamous,  or  single-chambered,  Foraminifer 
(Lagena.  striata) ;  b,  shell  of  a  polythalamous,  or  many-chambered,  Foraminifer 
{Polystomella.  crispa),  with  pseudopodia  extended  ;  c,  shell  of  a  Radiolarian,  one 
of  the  Polycystines  (Podocyrtzs  schomburgkii). 

innumerable  minute  orifices  (foramina)  through  which 
the  animal  protrudes  its  myriad  of  glairy,  threadlike 
arms.  The  majority  are  compound,  resembling  cham- 
bered cells,  formed  by  a  process  of  budding,  the  new 
cells  being  added  so  as  to  make  a  straight  series,  a 
spiral,  or  a  flat  coil.  As  a  rule,  the  many-chambered 
species  have  calcareous,  perforated  shells ;  and  the  one- 
chambered  have  an  imperforated  membranous,  porce- 
laneous,  or  arenaceous  envelope.  The  former  are  marine. 
There  are  few  parts  of  the  ocean  where  these  micro- 
scopic shells  do  not  occur,  and  in  astounding  numbers. 


PROTOZOA  59 

A  single  ounce  of  sand  from  the  Antilles  was  calculated 
to  contain  over  three  millions.  The  bottom  of  the  ocean, 
up  to  about  50°  on  each  side  of  the  Equator,  and  at 
depths  not  greater  than  2400  fathoms,  is  covered  with 
the  skeletons  of  these  animals,  which  are  constantly 
falling  upon  it  (Globigerina  ooze).  Their  remains  consti- 
tute a  great  proportion  of  the  so-called  sand  banks 
which  block  up  many  harbors.  Yet  they  are  descend- 
ants of  an  ancestry  still  more  prolific,  for  the  Foraminif era 
are  among  the  most  important  rock-building  animals. 
The  chalk  cliffs  of  England,  the  building  stone  of  Paris, 
and  the  blocks  in  the  Pyramids  of  Egypt  are  largely 
composed  of  extinct  Foraminifers.  Foraminifera  are 
both  marine  and  fresh-water,  chiefly  marine. 

The  sun  animalcule  (Actinophrys  sol\  one  of  the  Heli- 
ozoa,  is  common  in  the  slime  on  the  sides  of  aquaria. 
Its  spherical  body,  composed  of  frothy  protoplasm, 
bears  numerous  stiff  radiating  processes  by  means  of 
which  the  animal  moves  about  and  captures  its  food. 

A  Radiolarian  differs  from  a  Foraminifer  in  secreting 
a  siliceous,  instead  of  a  calcareous,  shell,  studded  with 
radiating  spines ;  and  in  the  central  part  of  the  body  is 
a  perforated  membranous  sac  containing  a  nucleus  or, 
sometimes,  several  nuclei.  The  most  of  the  protoplasm 
of  the  body  lies  outside  the  sac.  Radiolarians  are  more 
minute  than  Foraminifera,  but  as  widely  diffused.  They 
enter  largely  into  the  formation  of  some  strata  of  the 
earth's  crust,  and  abound  especially  in  the  rocks  of  Bar- 
badoes  and  at  Richmond,  Va.  The  living  forms  are 
marine. 

CLASS  2.  —  Mycetozoa 

These  organisms  are  frequently  classified  among  the 
plants  under  the  name  of  "slime  molds."  They  consist 
of  masses  of  protoplasm  of  various  sizes  and  colors,  and 


60       STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 


are  terrestrial  in  habit,  being  often  found  in  sum- 
mer slowly  crawling  upon 
stumps,  logs,  and  leaves. 
In  their  nonmotile  stage 
of  development  they  re- 
semble the  spore-bearing 
organs  and  spores  of  cer- 
tain Fungi,  but  in  their 
locomotor  phase  they  ex- 
hibit the  structure  and 
physiology  of  amoeboid 

FIG.  3. — Germination  of  a  spore  of  a  Myceto-  ,        n  11    4. 

zoan  (Trfchia),  showing  the  development      and       flagellate 
of  the  amoeboid  stage.     Much  magnified.        SOTTlPtimPS 

forming  large,  multinucleated  masses  of 
'protoplasm  which  crawl  about  and  ingest 
solid  food  (Fig.  3). 

CLASS  3.  —  Mastigophora 

The  distinctive  character  of  the  ani- 
mals in  this  group  is  the  presence  of 
one  or  more  flagella,  long,  whiplashlike 
threads  of  protoplasm  used  for  locomo- 
tion and  for  obtaining  food.  Some  kinds, 
like  Euglena  (Fig.  4),  live  as  independent 
organisms,  while  others,  as  Volvox  and 
Dinobryon,  form  colonies  (Fig.  5).  The 
latter  two  are  of  some  sanitary  impor- 
tance, since  either  one,  when  present 
in  large  numbers  in  a  water  supply,  is 
likely  to  cause  unpleasant  tastes  and 

.  FIG.  4.  — Euglena:  /, 

odors.     JVoctiluca,  a  marine  form,  is  one     flageiium;  g,  guiiet; 

r   ,  i  r        i  i  •        A!_          Pst  pigment  spot   or 

of  the  causes  of  phosphorescence  in  the     «eye";  cv,  contrac- 
sea.      Some    of    the    higher  kinds,  e.g.,     tiievacuoie:  r.reser- 

voir  ;  /,  paramylum 

Codosiga    (Fig;.   6\    are    interesting;    for    bodies;  r,  chlorophyll 

bodies.     Much  mag- 

the  reason  that  they  bear  a  peculiar  struc-    nified. 


PROTOZOA 


6l 


ture,  the  so-called  "  collar,"  which  is  found  practically 
nowhere  else  except  on  certain  cells  in  sponges  (Figs.  6, 


FIG.  5.  — Dinobryotti  portion 
of  the  motile  colony  show- 
ing zooids,  each  in  its  own 
lorica.  Much  magnified. 


FIG.  6.  —  Codosiga:  f,  fla- 
gellum  ;  c,  "  collar"  ;  bt 
body  ;  n,  nucleus  ;  cz>, 
contractile  vacuole  ;  nv, 
nutritive  vacuole.  Much 
magnified. 


'CLASS  4.  —  Sporozoa 
> 

The  Sporozoa  are  all  parasitic,  and  are  found  in  va- 
rious parts  of  the  bodies  of  fishes,  frogs,  turtles,  insects, 
crustaceans,  worms,  and  so  on,  some  living  in  the 
digestive  organs,  others  in  glands,  while  still  others 
penetrate  into  the  muscle  fibers  of  the  infested  animal 
(Fig.  7).  They  have  no  organs  of  locomotion,  but  move 
by  wormlike  contortions  of  the  body.  The  protoplasmic 
body  substance  is  covered  by  a  cuticle,  and  contains  a 
nucleus  (Fig.  8).  Liquid  food  is  absorbed  through  the 
cuticle  (Fig.  7). 


62       STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 

Probably  the  parasite  which  causes  malaria  in  man 
belongs  in  this  group. 


FIG.  7.  —  Sporozoa:  a,  embedded  in  the  striated  muscle  fiber  of  the  ox;  b,  Gregarina 
in  the  intestine  of  the  beetle  ;  c,  cyst  and  young  germs  of  Klossia  from  the  snail. 
All  much  magnified. 


Reproduction  is  by  means  of  spores.  The  individ- 
uals become  surrounded  by  a  thickened  covering  or  cyst 
secreted  from  their  bodies.  Within  this  cyst  division 
into  numerous  smaller  masses  takes  place.  Each  mass 


FIG.  8.  —  Gregarina  gigantea,  highly  magnified  :  a,  nucleus. 


then  secretes  a  thickened  coat  and  becomes  a  spore. 
When  mature,  the  protoplasmic  mass  within  breaks 
through  the  wall  of  the  cyst  and  enters  the  organ  or 
animal  in  which  the  parasite  reaches  its  adult  condition 
(Fig.  7> 


PROTOZOA 


CLASS  5.  —  Infusoria 

The  name  of  this  group  is  derived  from  the  fact  that 
the  animals  composing  it  are  almost  always  found  in 
infusions  of  vegetable  substance. 
The  characteristic  feature  of  the 
group  is  the  presence  of  fine,  hair- 
like  protrusions  of  the  body  sub- 
stance, which  more  or  less  com- 
pletely cover  the  animal,  and  which 
are  called  cilia.  These  are  perma- 
nent structures  in  some  forms 
(Ciliata),  but  are  found  only  in  the 
young  condition  of  others  (Tenta- 
culiferd),  the  adults  developing  -ten- 
tacles (Fig.  12).  Their  bodies  show 
a  great  variety  of  shapes,  spherical, 
flattened,  oval,  cylindrical,  conical, 
and  so  on.  Some  live  as  indepen- 
dent organisms,  as  Paramecium 
(Fig.  9);  others 
are  sedentary, 
being  attached 
by  a  stalk,  as 

Vorticella  (Fig.  n);  the  trumpet 
animal  (Stentor)  can  attach  itself  at 
will.  Epistylis  forms  branching  col- 
onies. In  some  colonies  the  mem- 
bers are  all  alike  in  structure  and 
function  (Carchesium),  while  in 
others  (Zoothamnium)  the  members 
which  capture  the  food  for  the 
FIG.  ™.- Paramecium  in  colony  are  plainly  different  in  shape, 
the  process  of  fission:  m,  sjze  an(^  structure  from  those  which 

mouth  ;     cv,     contractile 

vacuoie;«,macronucieus;     produce  the  new  colonies,  the  latter 

»',  micronucleus.      Much      ,  111  i  11 

magnified.  being  mouthless,  larger,  and  capable 


FIG.  9.  —  Paramecium,  c, 
cilia  ;  g,  gullet ;  f,  food 
vacuole  ;  t,  trichocysts  ; 
cv,  contractile  vacuole  ; 
m,  macronucleus  ;  n,  mi- 
cronucleus. Much  mag- 
nified. 


64       STRUCTURAL   AND   SYSTEMATIC  ZOOLOGY 


of  freeing  themselves  from  their  stalk  and  swimming 
away  to  another  place  where  the  new  colony  is  to  be 

started.  Thus  there  is 
shown  among  these  sim- 
ple organisms  the  differ- 
entiation of  parts  which 
is  one  of  the  character- 
istic features  of  the  higher 
forms  of  animal  life. 

The  cilia  are  used  for 
locomotion  and  for  ob- 
taining food,  which,  ex- 
cept in  the  parasitic 
/  species,  is  in  the  form 
of  solid  particles,  consist- 
ing of  bacteria  and  micro- 
scopic plants  and  animals, 
or  of  minute  fragments  of 
animal  and  vegetable  ma- 
terial. All  of  the  ciliata 
which  ingest  solid  food  have  a  permanent  mouth  open- 
ing. Tentaculifera 
suck  through  their 
tentacles  the  soft  ma- 
terial composing  the 
bodies  of  their  prey. 
The  cilia  are  uni- 
formly arranged  over 
the  body,  as  in  Para- 
mecium,  or  are  re- 
stricted to  definite 
regions,  as  in  Vorti- 
cella.  In  either  case 
there  may  be  variations  in  their  form,  size,  and  func- 
tion. Reproduction  is  by  division  and  by  budding, 


FIG.  ii. —  Vortice lla  :  a,  showing  stages  of 
fission,  and  b,  internal  structure;  d,  cili- 
ated disk  ;  g,  gullet  ;  «,  nucleus  ;  c,  con- 
tractile vacuole  ;  f,  food  vacuole.  Much 
magnified. 


FIG.  12.  —  Actneta,  animal  in  its  lorica,  /,  showing 
suctorial  tentacles  and  nucleus,  n,  with  contractile 
vacuole.  Only  a  small  part  of  the  stalk  is 
shown.  Magnified, 


PORIFERA  65 

spore  formation  being  exceptional.  It  has  been  esti- 
mated that  by  self-division  a  Parameciunt  may  give  rise 
to  1,364,000  in  forty-two  days  (Figs.  10,  n). 

SERIES    II.  — METAZOA 

The  Metazoa  include  all  those  animals  whose  bodies 
are  multicellular,  which  reproduce  by  true  eggs  and 
spermatozoa.  This  series  includes  eleven  of  the 
branches  of  the  animal  kingdom. 


Branch  II.  —  PORIFERA 

The  position  of  the  sponges  has  been  much  dis- 
puted. At  first  they  were  thought  to  be  on  the  border 
line  between  animals  and  plants,  and  were  assigned  by 
some  to  the  animals  and  by  others  to  the  vegetables. 
Later,  and  up  to  very  recent  years,  they  were  assigned 
to  the  Protozoa.  The  discovery  0 

of  their  mode  of  reproduction 
and  development  has  determined 
that  they  belong  to  the  Metazoa. 

Simple  sponges,  like  Grantia 
(Fig.  13),  are  somewhat  vase- 
shaped  in  outline,  and  have  a 
single  central  cavity  communi- 
cating with  the  outside  through 
an  opening  called  the  osculum. 
The  wall  of  the  body  is  pierced 
by  numerous  fine  canals  which 
communicate  more  or  less  di-  FlG-  X3- -Diagram  of  a  simple 

sponge:    t,  inhalant  opening; 

rectly  with  the  central  cavity  on  o,  exhaiant  opening  or  oscu- 
the  one  hand  and  with  the  exte- 
rior on  the  other.  There  is  no  body  cavity.  The 
body  wall  is  composed  of  the  skeleton,  together  with 
the  cellular  elements  forming  the  "flesh."  The  sur- 
DODGE'S  GEN.  ZOOL.  —  5 


66       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


face  of  the  body  is  covered  by  a  single  layer  of  flat- 
tened cells  forming  the  ectoderm.  The  canals  are  more 
or  less  completely  lined  with  a  layer  of  cells,  each 
of  which  is  provided  with  &  flag e  Hum,  by  means  of  which 
water  is  propelled  through  the  canals  toward  the  central 
cavity.  Between  these  two  layers  is  a  mass  of  amoeboid 
and  other  cells  which  compose  the  mesoderm,  and  in 
which  the  skeleton,  or  framework,  of  the  sponge  is  de- 
veloped. The  skeleton  may  be  composed  of  flexible 
fibers  of  spongin,  as  in  the  toilet  sponge ; 
or  of  spongin  fibers  together  with  spic- 
ules  of  calcareous  matter,  as  in  Grantia  ; 
or  of  siliceous  spicules  alone,  as  in 
the  fresh-water  sponge  (Spongilla  and 
Myenia)  and  Venus's  flower  basket 
(Euplectelld) ',  or  of  spicules  of  carbo- 
nate of  lime  and  so  on,  while  a  few 
have  no  skeleton  at  all. 

The  flagellate  cells  are  peculiar,  in 
that  they  have  an  upgrowth  on  their 
free  end,  formed  by  a  delicate  expansion 
of  the  cell  substance,  and  having  the 
shape  of  a  broad  collar  surrounding  the 
base  of  the  flagellum,  whence  their 
name  of  "collar  cell"  (Fig.  14). 

The  water  flowing  in  through  the 
canals  bears  with  it  the  small  particles 
of  organic  material  upon  which  the  sponge  feeds,  the 
particles  being  captured  apparently  by  the  collar  cells 
as  well  as  by  the  amoeboid  cells.  The  same  currents 
of  water  serve  also  for  respiration.  Reproduction  is  by 
means  of  eggs  and  by  budding,  the  latter  process  giving 
rise  to  a  group  of  connected  sponges.  The  young  larval 
sponge  which  develops  from  the  fertilized  egg  is  pro- 
vided with  cilia,  by  means  of  which  it  can  swim  around 


FIG.  14.  —  "Collar 
cell "  from  Grantia, 
showing  flagellum, 
f;  "  collar,"  c  ;  nu- 
cleus, n ;  contractile 
vacuole,  cv .  Much 
magnified. 


PORIFERA 


67 


for  a  time.     Later,  it  comes  to  rest,  attaches  itself  to 
some  support,  and  develops  into  the  adult  form  which  is 

d 

h 


FIG.  15. — Hypothetical  Section  of  a  Sponge:  a,  superficial  layer  ;  b,  inhalant  pores; 
c,  ciliated  chambers  ;  d,  exhalant  aperture,  or  osculum  ;  e,  deeper  substance  of  the 
Sponge. 

never  capable  of  locomotion.     The  fresh-water  sponges 
also  multiply  by  means  of  gemmules,  which  are  small, 


FIG.  16.  —  Skeleton  of  a  Horny  Sponge. 

seedlike  bodies  to  be  found  in  the  sponge  in  the  fall. 
Each  consists  of  a  hard  coating  surrounding  a  mass  of 


68       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

cells  and  food  substance.  These  gemmules  survive  the 
winter,  and.in  the  spring  the  cellular  contents  come  out 
and  develop  into  a  sponge. 

The  sponge  individual  contains  one  exhalant  orifice 
(osculum),  with  the  channels  leading  into  it.  An  ordi- 
nary bathing  sponge  constitutes  a  colony  of  such  indi- 
viduals, which  are  not  definitely  marked  off  from  each 
other.  Some  other  sponges  have  only  one  osculum,  and 
such  are  a  single  individual,  e.g.  Grantia. 

Excepting  a  few  small  fresh-water  species  (as  Spon- 
gilla\  sponges  are  marine.  In  the  former,  the  cellular 
part  is  greenish,  containing  chlorophyll ;  in  the  latter,  it 
is  brown,  red,  or  purple.  In  preparing  the  sponge  of 
commerce,  this  is  rotted  by  exposure,  and  washed  out. 
The  best  fishing  grounds  are  the  eastern  end  of  the 
Mediterranean  and  around  the  Bahama  Islands. 


Branch   III.  —  CCELENTERATA 

In  the  animals  comprising  this  group,  the  body 
cavity  is  not  distinctly  separated  from  the  digestive 
cavity.  The  ccelenterates  are  almost  wholly  marine 
forms, — hydroids,  corals,  sea  anemones  and  jellyfishes, — 
but  there  are  a  few  which,  like  Hydra,  live  in  fresh 
water.  The  body  is  usually  radially  symmetrical  and 
shows  three  more  or  less  definite  cell  layers,  the  ecto- 
derm on  the  outer  surface,  the  endoderm  lining  the  inner 
cavities,  with  the  mesoderm,  or  middle  layer,  between 
the  others.  In  hydra  and  the  hydroids  the  mesoderm 
is  reduced  to  a  mere  film,  but  in  the  jellyfishes  and 
sea  anemones  it  forms  a  large  part  of  the  body.  A 
characteristic  feature  is  the  presence  of  the  stinging 
cells,  or  nematocysts,  which  are  almost  invariably  to  be 
found  except  in  one  group,  —  Ctenophora,  —  where  they 
are  replaced  by  adhesive  cells. 


CCELENTERATA  69 

This  branch  consists  of  two  rather  divergent  forms, 
represented  on  the  one  hand  by  the  sedentary  type 
(hydroid),  and  on  the  other  by  the  free-swimming  type 
(jellyfish).  These  two  forms  may  occur  during  the 
course  of  development  of  one  individual,  thus  illustrat- 
ing the  phenomenon  of  "alternation  of  generations." 
Many  of  the  members  of  the  group  are  soft-bodied, 
while  others  secrete  calcareous  material  forming  coral. 

All  of  the  coelenterates  multiply  by  means  of  eggs 
and  sperm  cells,  and  all,  except  the  Ctenophora,  by 
budding  as  well,  the  latter  method  resulting  in  the 
formation  of  colonies  in  which  the  various  members 
often  differ  greatly  in  form  and  in  function. 

The  animals  in  this  group  are  carnivorous,  feeding 
mainly  on  small  organisms,  although  the  sea  anemone 
can  ingulf  masses  of  considerable  size. 

There  are  four  classes  :  — 

Class  i.    Hydrozoa,    represented    by    hydra     and    the 

hydroids. 
Class  2.    Scyphozoa,    containing    the   large    jellyfishes, 

for  example,  Aurelia. 
Class  3.    Actinozoa,  including  the  sea  anemone  and  the 

corals. 
Class  4.    Ctenophora,  including    the   jellyfishes    which 

have  comblike  swimming  organs. 

CLASS  i .  —  Hydrozoa 

In  these  coelenterates  the  body  is  a  simple  tube,  or 
cavity,  in  which  there  is  a  single  aperture,  the  mouth. 
The  nervous  system  is  slightly  developed.  Such  are 
fresh-water  hydras  and  the  oceanic  hydroids  (Eucope). 

The  body  of  the  hydra  is  tubular,  soft,  and  sensitive, 
of  a  greenish  or  brownish  color,  and  seldom  over  half 
an  inch  long.  It  is  found  spontaneously  attached  by 


70       STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 


one  end  to  submerged  plants,  while  the  free  end  con- 
tains the  orifice,  or  mouth,  crowned  with  tentacles,  by 
which  the  creature  feeds  and  creeps.  The  body  wall 
consists  of  two  cellular  layers  —  ectoderm  and  endo- 
derm.  These  surround  a  central  cavity  with  one 
opening.  The  animal  may  be  compared  to  a  bag  with 

a  two-layered  wall,  and 
with  tentacles  around 
the  opening.  It  buds, 
and  also  reproduces  by 
eggs.  The  buds,  when 
adult,  become  detached 
from  the  parent  (Figs. 
17,  1 8). 

In  most  of  the  other 
Hydrozoa  the  colony 
is  permanent,  and  sup- 
ported by  a  horny 
skeleton.  There  are 
two  kinds  of  Polyps  in 
each  colony,  one  for 
feeding  and  the  other 

FIG.  17.  — Hydra:  2,  with  tentacles  fully  extended;    for    reproduction    (Fig. 
3,  creeping  ;  4,  with  ingested  prey  :  5,  budding.  ~  .  - 

20).       Sometimes    the 

reproductive  Polyps  are  separated  from  the  stock 
in  the  form  of  little  jellyfishes,  and  are  then  called 
medusae  (Figs.  20  m,  21).  Belonging  to  this  class 
are  Hydractinia,  found  on  the  shells  inhabited  by  the 
hermit  crab ;  the  elk-horn  coral  (Millepora) ;  and  the 
beautiful  Portuguese  man-of-war,  consisting  of  a  bladder- 
like  float  from  the  bottom  of  which  depend  tentacles 
many  feet  in  length  and  several  kinds  of  polyps,  the 
tentacles  being  covered  with  stinging  cells,  which  aid 
in  capturing  the  prey  and  in  defending  the  colony. 


CGELENTERATA 


FIG.  18.  —  Hydra:  longitudinal  section  of  ani-  FIG.     20.  —  Campanularian     hydroid: 

mal   showing   mt  mouth;    /,  tentacle;    d,  portion  of  colony,  showing  nutritive 

digestive  cavity;    b,  bud;    s,  spermary  ;  zooids,  _/;    reproductive  zooid,    r  ; 

o,  ovary  ;    ec,  ectoderm  ;    en,  endoderm.  young  zooid,  y  ;    and  medusa,  m 

Magnified.  Magnified. 


FIG.  19.  —  Hydroid  (Sertularia)  growing  on  a  shell. 


FIG.  21. —  A  Medusa,  seen  in 
profile  and  from  below, 
showing  central  manu- 
brium,  radiating  and  mar- 
ginal canals  and  tentacles. 


72       STRUCTURAL   AND   SYSTEMATIC  ZOOLOGY 

CLASS  2.  —  Scyphozoa 

These  are  jellyfishes  which  are  characterized  mainly 
by  having  reproductive  glands  which  discharge  their 
contents  into  the  stomach  whence  the  reproductive  cells 
make  their  way  out  through  the  mouth,  by  having  the 
gastric  cavity  much  branched  and  ramifying  through 
the  gelatinous  body,  and  by  the  presence  of  filaments 
projecting  into  the  gastric  cavity. 


FIG.  22  —  Jellyfish  (Pelagia  noctiluca).     Mediter- 
ra  nean. 


FIG.  23.  —  Portuguese  Man- 
of-war  (Physalia),  \  nat- 
ural size.  Tropical  Atlantic. 


The  jellyfish  has  a  soft,  gelatinous,  semitransparent, 
umbrella-shaped  body,  with  tubes  radiating  from  the 
central  digestive  cavity  to  the  circumference,  and  with 
the  margin  fringed  with  tentacles,  which  are  furnished 


CCELENTERATA 


73 


with  stinging  thread  cells.  The  radiating  parts  are  in 
multiples  of  four.  Around  the  rim  are  minute  colored 
spots,  the  "eye  specks."  In  fine  weather,  these  "sea 
blubbers"  are  seen  floating  on  the  sea,  mouth  down- 
ward, moving  about  by  flapping  their  sides,  like  the 
opening  and  shutting  of  an  umbrella,  with  great 
regularity.  They  are  frequently  phosphorescent  when 
disturbed.  Some  are  quite  small,  resembling  little  glass 
bells ;  the  common  Aurelia  is  over  a  foot  in  diameter 
when  full-grown  ;  while  the  Cyanea,  the  giant  among 
jellyfishes,  sometimes  measures  eight  feet  in  diameter, 
with  tentacles  more  than  one  hundred  feet  long. 
The  tissues  are  so  watery  that,  when  dried,  nothing 
is  left  but  a  film  of  membrane  weighing  only  a  few 
grains. 

The  two  common  types  are  Lucernaria  and  Aurelia. 
The  former  is  the  Umbrella-acaleph  and  has  a  short 
pedicel  on  the  back  for 
voluntary  attachment ; 
tentacles  disposed  in 
eight  groups  around 
the  margin,  the  eight 
points  alternating  with 

the     four    partitions     Of     FIG.  z^.—Lucernaria   auricula   attached   to  a 

piece    of   seaweed;    natural  size.     The  one  on 
the  body  CaVlty  and  the  the  right  is  abnormal,  having  a  ninth  tuft  of 

four    corners     of '  the       tentacles' 

mouth;  not  less  than  eight  radiating  canals,  and  no 
membranous  veil.  The  common  species  on  the  Atlantic 
shore,  generally  found  attached  to  eelgrass,  is  an  inch 
in  diameter,  of  a  green  color.  Aurelia,  the  ordinary 
jellyfish,  is  free  and  oceanic.  It  differs  from  the  Lucer- 
naria in  its  usually  larger  size  and  solid  disk,  and  in 
Having  four  radiating  canals,  which  ramify  and  open 
into  a  circular  vessel  which  runs  around  the  margin  of 
the  disk.9 


74       STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 


FlG.  25.  —  Horizontal  Section  of  Actinia 
through  the  stomach,  showing  septa 
or  mesenteries,  and  compartments. 


CLASS  3.  —  Actinozoa   - 

These  marine  animals,  which  by  their  gay  tentacles 

convert  the  bed  of  the  ocean  into  a  flower  garden,  or  by 

their  secretions  build  up  coral 
islands,  have  a  body  like 
a  cylindrical  gelatinous  bag. 
One  end,  the  base,  is  usually 
attached;  the  other  has  the 
mouth  in  the  center,  sur- 
rounded by  numerous  hollow 
tentacles,  which  are  covered 
with  nettling  lasso  cells.  This 
upper  edge  is  turned  in  so  as 
to  form  a  sac  within  a  sac, 
like  the  neck  of  a  bottle 

turned  outside  in.    The  inner  sac,  which  is  the  digestive 

cavity,  does  not  reach  the  bottom,  but  opens  into  the 

general    body   cavity 

(Fig.     236).10       The 

space  between  these 

two  concentric  tubes 

is  divided  by  a  series 

of  vertical    partitions 

or  mesenteries,  some 

of  which  extend  from 

the  body  wall  to  the 

digestive  sac,  but  oth- 
ers fall  short  of  it. 

Instead,  therefore,  of 

the    radiating     tubes 

of    the    Scyphozoan, 

there    are     radiating 

spaces.     No  members  of  this  class  are  microscopic.     All 

are  long-lived  compared  with  the  Hydrozoa,  living  for 


FIG.  26.  —  Actinia  expanded,  seen  from  above, 
showing  mouth. 


CCELENTERATA  75 

several  years.     One  kept  in  an  aquarium  in  England 
lived  to  be  more  than  sixty  years  old. 
There  are  two  subclasses  :  — 

1.  Zoantharia,  including  the  sea  anemones  and  the 
stony  corals,  and, 

2.  Alcyonaria,  to  which  belong  the  organ-pipe  coral 
(Tubipord),  sea  fan  (Gorgonia\  the  precious  red  coral 
(Corallium),  and  the  sea  pen  (Pennatula). 

Zoantharia  usually  have  numerous  tentacles,  generally 
arranged  in  multiples  of  five  or  six,  the  tentacles  being 
unbranched  and  hollow,  while  in  the  Alcyonaria  the 
tentacles  are  finely  branched  and  are  always  eight  in 
number. 

Zoantharia. — The  best-known  representative  of  this 
group  is  the  Metridium,  or  sea  anemone.  It  usually 
leads  a  solitary  life,  though  frequently  several  are  found 
together,  some  of  which  have  arisen  as  buds  from  the 
others.  It  is  capable  of  a  slow  locomotion.  Muscular 
fibers  run  around  the  body,  and  others*  cross  these  at 
right  angles.  The  tentacles,  which  often  number  over 
two  hundred,  and  the  partitions,  which  are  in  reality 
double,  are  in  multiples  of  six.  At  night,  or  when 
alarmed,  the  tentacles  are  drawn  in,  and  the  aperture 
firmly  closed,  so  that  the  animal  looks  like  a  rounded 
lump  of  fleshy  substance  plastered  on  the  rock.  It 
feeds  on  crabs  and  mollusks.  It  abounds  on  every 
shore,  especially  of  tropical  seas.  The  size  varies  from 
one  eighth  of  an  inch  to  a  foot  in  diameter  (Fig.  236). 

Alcyonaria.  — The  most  of  the  animals  in  this  group 
grow  in  branching  colonies,  the  axis  consisting  of  a 
horny  substance  covered  with  flesh  in  which  spicules 
of  lime  are  found.  The  polyps  are  usually  small. 
The  sea  pen  (Pennatula}  grows  with  one  end  em- 
bedded in  the  mud  and  sand  of  the  sea  bottom.  In 
Gorgonia,  the  sea  fan,  the  branches  arise  in  the  same 


76       STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 


vertical  plane  and  unite  to  form    a  beautiful  network 

(Fig.  35). 

Coral. — The  majority  of  Actinozoa  secrete  a  calca- 
reous or  horny  framework  called  "  coral."  With  few 
exceptions,  they  are  fixed  and  composite,  living  in  colo- 
nies formed  by  a  continuous 
process  of  budding.  Their  struc- 
tures take  a  variety  of  shapes ; 
often  domelike,  but  often  resem- 
bling shrubbery  and  clusters  of 
leaves.  The  members  of  a  coral 
community  are  organically  con- 
nected; each  feeds  himself,  yet 
is  not  independent  of  the  rest. 
The  compound  mass  is  "  like  a 
living  sheet  of  animal  matter, 
fed  and  nourished  by  numerous 
mouths  and  as  many  stomachs." 
•-  Life  and  death  go  on  together, 
the  old  polyps  dying  below  as 
new  ones  are  developed  above.  The  living  part  of  an 
Astrcea  is  only  half  an  inch  thick.  The  growth  of  the 
branching  Madrepora  is  about  three  inches  a  year.  The 
colors  of  the  coral  polyps  are  brilliant  and  varied,  being 
green,  purple,  pink,  or  brown.  The  organ-pipe  coral  has 
green  polyps  and  crimson  skeleton,  while  the  precious 
£,sx2\(Corallium)  has  white  polyps  and  a  bright  red  axis. 
Another  kind  is  bright  blue.  The  usual  size  varies 
from  that  of  a  pin's  head  to  half  an  inch,  but  the 
mushroom  coral  (which  is  a  single  individual)  may  be 
a  foot  in  diameter. 

Corals  are  of  two  kinds :  those  deposited  within  the 
tissues  of  the  animal  (sclerodermic\  and  those  secreted 
by  the  outer  surface  at  the  foot  of  the  polyp  (sclero- 
basic).  The  polyps  producing  the  former  are  actinoid, 


FIG.  27.  —  Organ-pipe  coral  (Tu 

pora  ntusica).     Indian  Ocean. 


CCELENTERATA  77 

resembling  the  Actinia  in  structure.11  The  skeleton  of 
a  single  polyp  (called  corallite,  Fig.  292)  is  a  copy  of 
the  animal,  except  the  stomach  and  tentacles,  the  earthy 
matter  being  secreted  within  the  outer  wall  and  between 
each  pair  of  partitions.  So  that  a  corallite  is  a  short 
tube  with  vertical  septa  radiating  toward  the  center.12 


FIG.  28.  — Madrepora  aspera,  living  and  expanded  ;  natural  size.     Pacific. 

« 

A  sclerobasic  coral  is  a  true  exoskeleton,  and  is  dis- 
tinguished by  being  smooth  and  solid.  The  polyps, 
having  eight  fringed  tenacles,  are  situated  on  the  out- 
side of  this  as  a  common  axis,  and  are  connected  to- 
gether by  the  fleshy  ccenosarc  covering  the  coral. 

(i)  Sclerodermic  Corals. — '  Astrcea  is  a  hemispherical  mass 
covered  with  large  cells.     Meandrina,  or  "brain  coral," 


78       STRUCTURAL  AND    SYSTEMATIC  ZOOLOGY 

is  also  globular ;  but  the  mouths  of  the  polyps  open  into 
each  other,  forming  furrows.  Fungia,  or  "  mushroom 
coral,"  is  disk-shaped,  and  differs  from  other  kinds  in 


FIG.  29.  —  Ctenactis  ecktnata,  or  "  Mushroom  Coral " :  one  fourth  natural  size.     Pacific. 

being  the  secretion  of  a  single  gigantic  polyp,  and  in 
not  being  fixed.      Madrepora  is  neatly  branched,  with 


FIG.  30.  —  A strcea pallida,  living  colony;   natural  size.     Fejee  Islands. 

pointed  extremities,  each  ending  in  a  small  cell  about 
a  line  in  diameter.  Porites,  or  "sponge  coral,"  is  also 
branching,  but  the  ends  are  blunt,  and  the  surface  com- 


CCELENTERATA 


79 


paratively  smooth.  Tubipora,  or  "  organ-pipe  coral," 
consists  of  smooth  red  tubes  connected  at  intervals  by 
cross  plates.  The  Astrcea,  Meandrina,  Madrepom,  and 
Porites  are  the  chief  reef-forming  corals.  They  will 
not  live  in  waters  whose  mean  temperature  in  the 
coldest  month  is  below  68°  Fahr.,  nor  at  greater  depth 
than  about  twenty  fathoms.  The  most  luxuriant  reefs 


FIG.  31.  —  Diploria  cerebriformis,  or  "  Brain  Coral " ;  one  half  natural  size.    Bermudas. 


are  in  the  central  and  western  Pacific  and  around  the 
West  Indies. 

A  coral  reef  is  formed  by  many  corals  growing  to- 
gether. It  is  to  the  single  coral  stock  as  a  forest  is  to 
a  tree.  The  main  kinds  of  reefs  are  fringing,  where 
the  reef  is  close  to  the  shore ;  barrier,  where  there  is  a 
channel  between  reef  and  shore;  encircling,  where  there 
is  a  small  island  inside  of  a  large  reef ;  and  coral  islands, 


80       STRUCTURAL  AND   SYSTEiMATIC   ZOOLOGY 


or  atolls,  where  there  is  simply  a  reef  with  no  land  in- 
side of  it.     The  Great  Barrier  Reef  off  the  east  coast  of 


FIG.  32. —  Astrcza  rotulosa.     West  Indies. 


-' 


FIG.  33.  —  Cell  of  Madrepore  Coral, 
magnified.  The  cuplike  depres- 
sion at  the  top  of  a  coral  skele- 
ton is  called  calicle. 


FIG.  34.  —  Fragment  of  Red  Coral  (Coral- 
Hum  rubrutii) ,  showing  living  cortex 
and  expanded  polyps.  Mediterranean. 


Australia  is  1250  miles  in  length.     All  reefs  begin  as 
fringing  reefs,  and  are  gradually  changed  into  the  other 


CCELENTERATA 


8l 


forms  by  the  slow  sinking  of  the  bottom  of  the  ocean, 
or  by  the  death,  decay,  and  disintegration  of  the  corals 
on  the  landward  side  of  the  reef,  where  the  food  supply 
is  necessarily  restricted. 


FIG.  35.  —  Sea  Fan  {Gorgonia}  and  Sea  Pen  (Pennatula}. 

(2)  Sclerobasic  Corals. —  Corallium  rubrum,  the  precious 
coral  of  commerce,  is  shrublike,  about  a  foot  high,  solid 
throughout,  taking  a  high  polish, 
finely  grooved  on  the  surface,  and 
of  a  crimson  or  rose-red  color. 
In  the  living  state  the  branches 
are  covered  with  a  red  ccenosarc 
studded  with  white  polyps  (Fig. 
34). 

CLASS  4.  —  Ctenophora 

The  CtenopJiora  (as  the  Pleuro- 
brachia,  Cestum,  and  Beroe)  are 
transparent  and  gelatinous,  swim- 
ming on  the  ocean  by  means  of  FIG.  36.  — A  ctenophore 

,  ..  robrachia  pileus);    natural 

eight    comblike,    ciliated    bands,        size. 
DODGE'S  GEN.  ZOOL.  —  6 


82       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

which  work  like  paddles.  The  body  is  not  contractile, 
as  in  the  jellyfishes.  They  are  considered  the  highest  of 
coelenterates,  having  a  complex  nutritive  apparatus  and 
a  definite  nervous  system.  There  is  no  trace  of  a  polyp 
stage  in  their  development,  and  they  do  not  form  colo- 
nies. They  are  found  in  all  regions  of  the  ocean,  from 
the  arctics  to  the  tropics. 

Branch  IV.  —  PLATYHELMINTHES 

The  group  formerly  called  Vermes  or  worms  was 
composed  of  animals  so  very  different  in  form  and 
structure  that  it  has  now  been  subdivided  into  several 
branches,  viz.  :  Platyhelminthes,  or  flat  worms ;  Ne- 
mathelminthes,  or  round  worms  ;  Trochelminthes ,  or 
rotifers ;  Molluscoida,  including  the  Polyzoa  and  Bra- 
chiopoda ;  and  Annulata,  or  segmented  worms.  All 
these  forms  agree  in  being  distinctly  bilaterally  sym- 
metrical animals,  as  contrasted  with  the  apparently 
radial  arrangement  of  parts  seen  in  the  C&lenterata 
and  EC  kino  derma  ta,  and  in  having  the  three  body  layers 
— ectoderm,  mesoderm,  and  endoderm  —  well  developed, 
the  mesoderm  or  middle  layer  being  relatively  of  more 
importance  than  in  any  preceding  group. 

The  Platyhelminthes,  or  flat  worms,  include  some  free 
forms,  as  Planaria,  which  is  common  in  fresh  water, 
and  the  tapeworms  and  flukes  among  the  parasites. 
As  a  group,  they  are  soft-bodied,  flattened  animals, 
without  skeletal  parts  of  any  kind.  There  is  no  distinct 
body  cavity  nor  blood-vascular  system  nor  anal  opening. 
The  digestive  system  may  be  entirely  absent,  as  in  the 
tapeworm,  or  it  may  be  much  branched  and  highly 
complicated  in  structure,  as  in  the  planarians. 

The  tapeworm  (  Tcenia)  consists  of  the  so-called  head 
and  the  body  segments,  which  are  really  reproductive 


PLATYHELMINTHES 


joints.  It  develops  from  the  egg  in  the  digestive  canal 
of  the  pig,  burrows  into  the  muscular  tissue  of  the 
animal,  and  there  becomes  encased.  Pork  containing 

these  cysts  is  called 
"measly  pork."  If 
the  pork  be  eaten 
by  man,  in  an  un- 
cooked condition, 
this  case  is  dis- 
solved by  the  gas- 
tric juice,  and  the 
embryo  thus  re- 
leased attaches  it- 
self to  the  intes- 


FIG.  37.  — Tapeworm  (Ttettia  soliunt) :  a,  bead;  b,  c,  d, 
segments  of  the  body. 


FIG.  38.  —  Planarian 
Worm. 


tine  by  its  "  head,"  and  develops  into  the  tapeworm  by 
budding  off  the  reproductive  segments,  or  proglottides. 
As  these  become  ripe  and  filled  with  fertilized  eggs, 
they  are  detached,  and  pass  off  with  the  excrement. 

The  disease  called  "  rot,"  in  sheep,  is  produced  by  the 
fluke  (Distoma),  which  grows  in  the  bile  ducts  of  the 
sheep. 

The  flat  worms  are  the  most  widely  distributed  of  all 
animals  above  the  Protozoa.  They  are  found  on  land, 
at  various  depths  in  bodies  of  fresh  water,  and  in  the 
sea.  They  also  occur  as  parasites  in  animals  in  almost 
every  class  of  the  Metazoa. 


84       STRUCTURAL  AND   SYSTEMATIC  ZOOLOGY 


Branch  V.  —  NEMATHELMINTHES 

The  round,  or  thread,  worms  include  free  forms,  as 
the  vinegar  eel ;  parasitic  forms,  as  the  pin  worm 
(Ascaris)  and  trichina ;  and  forms  free  when  adult,  and 
parasitic  when  young,  as  the  hair  worm  ( Gordius).  The 
body  is  usually  elongated  and  cylindrical  in  shape, 
whence  the  name.  In  most  forms  there  are 
plainly  marked  digestive  and  nervous  systems. 
The  trichina  is  usually  derived  by  man  from 
11  the  flesh  of  the  pig.  It 

exists  in  the  muscles, 
inclosed  in  micro- 
scopic cases  or  cysts, 
composed  of  calcareous 
matter.  If  the  meat  be 
eaten  uncooked  or  par- 
tially cooked,  the  cases 
are  dissolved,  and  the 
trichinae  become  sexu- 
ally mature  in  the  in- 
testines. The  young 

FIG.    •&.- Trichina    spiralis    (much   enlarged):    are  pro(Juced  and  bur- 
I,   male;    a,   mouth;    c,   intestine;  II,  capsules, 
with  trichinae  in  muscle.  TOW  their  Way  into  the 

muscles,  usually  of  the  back  and  limbs,  where  they  be- 
come encysted  in  the  muscle  fibers.  In  burrowing  they 
cause  great  pain  and  fever,  and  sometimes  death.  The 
adult  trichina  is  about  -^  of  an  inch  long. 

The  " jiorse-hair  snake,"  a  hair  worm  {Gordius), 
passes  the  early  part  of  its  existence  in  larval  or  adult 
insects,  e.g.,  the  cricket.  When  mature  the  worms  leave 
the  body  of  the  insect  and  lay  their  eggs  in  damp  places. 
The  eggs  or  the  immature  worms  are  then  taken  into 
the  bodies  of  other  insects  in  which  the  parasites  later 
reach  their  full  development. 


MOLLUSCOIDA 


Branch  VI.  • —  TROCHELMINTHES 

The  wheel  animalcules,  or  rotifers,  mostly  found  in 
fresh  water,  are  composed  of  a  few  ill-defined  segments, 
and  have  on  the  anterior  end  a  disk 
which  is  ciliated  on  the  edge,  the 
motion  of  the  cilia  causing  the  ap- 
pearance of  a  rotating  wheel,  whence 
the  name.  They  are  from  200  to  WQ 
of  an  inch  long.  They  have  a  well- 
developed  digestive  system,  the  food 
consisting  of  minute  organisms,  and 
a  rudimentary  nervous  system.  Roti- 
fers have  been  kept  for  several  years 
in  a  dried  condition  and  have  after- 
ward been  revived  (Fig.  40). 


Branch  VII. 


FIG.  40.  -r  Rotifer,  or 
''  Wheel  animalcule  " 
(Hydatina},  highly 
magnified. 


MOLLUSCOIDA 

These  ani- 
mals have  gen- 
erally a  body 
cavity,  in  which  lies  the  alimen- 
tary canal,  bent  in  such  a  man- 
ner that  the  mouth  and  the  anal 
opening  are  close  together.  Near 
the  mouth  is  a  curved  ridge, 
the  lophophore,  bearing  tentacles. 
There  is  a  very  rudimentary 
nervous  system  (Fig.  41). 

The     PolyZOa    rCSCmble     polyps 


FiG.4i.-DiagramofaPoly2oan: 

Aiophophore  bearing  tentacles,  jn  appearance,  living  in  clusters, 

//  m,  mouth;  a,  digestive  cav-  •  ... 

ity;    i,  intestine:  a,  anus;    e,  each   individual    inhabiting   a   dell- 

excretory  organ;    b,  "brain."  ,,  ,  -.     , 

Much  magnified.  cat  e  cell,  or  tube,  and  having  a 


86       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

simple  mouth  surrounded  with  ciliated  tentacles.  The 
colony  often  takes  a  plantlike  form ;  sometimes  spreads, 
like  fairy  chains  or  lacework,  over  other  bodies ;  or 
covers  rocks  and  seaweeds  in  patches  with  a  delicate 
film.  The  majority  secrete  carbonate  of  lime.  A  poly- 
zoan  shows  its  superiority  to  the  coral,  which  it  resem- 
bles, in  possessing  a  distinct  alimentary  canal  and  a 
nervous  system.  The  cells  of  a  group  are  never  con- 


FIG.  42. —  Polyzoans  :  i.    Hornera  lichenoides,  natural  size.     2.  Branch  of  the  same; 
magnified.     3.  Discopora  Skenei,  greatly  enlarged. 


nected  by  a  common  tube,  as  in  coelenterates.     There 
are  both  marine  and  fresh-water  species. 

The  Brachiopoda  or  "lamp  shells"  have  a  bivalve 
shell,  the  valves  being  applied  to  the  dorsal  and  ventral 
sides  of  the  body.  The  valves  are  unequal,  the  ventral 
being  usually  larger,  and  more  convex ;  but  they  are 
symmetrical,  i.e.,  a  vertical  line  let  fall  from  the  hinge 
divides  the  shell  into  two  equal  parts.  The  ventral 
valve  has,  in  the  great  majority,  a  prominent  beak,  per- 
forated by  a  foramen,  or  hole,  through  which  a  fleshy 
stalk  protrudes  to  attach  the  animal  to  submarine  rocks. 


ECHINODERMATA  87 

The  valves  are  opened  and  shut  by  means  of  muscles, 
and  in  most  cases  they  are  hinged,  having  teeth  and 
sockets  near  the  beak.  The  mouth  faces  the  middle  of 
the  margin  opposite  the 
beak  ;  and  on  either  side  of 
it  is  a  long  fringed  "  arm," 
generally  coiled  up,  and 
supported  by  a  calcareous 
framework.  The  animal, 


FIG.  43.  —  A  Brachiopod 
(Terebratulina  septen- 
trionalis} .  Atlantic  coast. 


FIG.  44.  —  Dorsal  Valve  of  a  Brachiopod 
(Terebratula) ,  showing,  in  descending 
order,  cardinal  process,  dental  sockets, 
hinge  plate,  septum,  and  loop  supporting 
the  ciliated  arms. 


having  no  gills,  respires  by  the  arms  and  the  mantle. 
Brachiopods  were  once  very  abundant,  over  two  thou- 
sand extinct  species  having  been  described;  but  only 
about  a  hundred  species  are  now  living.13  These  are  all 
marine,  and  fixed.  The  animals  in  this  group  are 
related  to  the  mollusca. 


Branch  VIII.  —  ECHINODERMATA 

The  echinoderms,  as  starfishes  and  sea  urchins,  are 
characterized  by  the  possession  of  a  distinct  nervous 
system  (a  ring  around  the  mouth  with  radiating  branches); 
an  alimentary  canal,  completely  shut  off  from  the  body 
cavity,  having  both  oral  and  anal  apertures ;  a  water- 
vascular  system  of  circular  and  radiating  canals,  con- 
nected with  the  outside  water  by  means  of  the  madre- 
poric  tubercle,  and  a  symmetrical  arrangement  of  all  the 


88       STRUCTURAL  AND   SYSTEMATIC  ZOOLOGY 

parts  of  the  txody  around  a  central  axis  in  multiples  of 
five,14  this  radial  arrangement,  however,  concealing  a 
definite  bilateral  symmetry.  They  are,  thus,  much 
more  highly  organized  than  the  coelenterates,  with 
which  group  they  have  very  little  in  common  except 


FIG.  45.  —  Forms  of  Echinoderms,  from  radiate  to  annulose  type:   a,  Crinoids:  b,  Ophi- 
urans;  c,  Starfish;  d,  Echini;  e,  Holothurians. 

their  apparent  radial  symmetry.  In  the  course  of 
development  in  echinoderms  metamorphosis  occurs,  the 
larval  forms  bearing  no  resemblance  to  the  adults. 

There  are  five  principal  classes,  all  exclusively  marine 
and  solitary,  and  all  having  the  power  of  secreting  more 
or  less  calcareous  matter  to  form  the  skeleton. 

CLASS  i.  —  Asteroidea 

Ordinary  starfishes  consist  of  a  flat  central  disk,  with 
five  or  more  arms,  or  lobes,  radiating  from  it,  and  con- 
taining branches  of  the  viscera.  The  skeleton  is  leathery, 
hardened  by  small  calcareous  plates  (twelve  thousand  by 
calculation),  but  somewhat  flexible.  The  mouth  is  below ; 
and  the  rays  are  furrowed  underneath,  and  pierced  with 


ECHINODERMATA  89 

numerous  holes,  through  which  pass  the  suckerlike  tenta- 
cles— the  organs  of  locomotion  and  prehension.  The  red 
spots  at  the  ends  of  the  rays  are  eyes.  The  usual  color 
of  starfishes  is  yellow,  orange,  or  red.  They  abound 
on  every  shore,  and  are  often  seen  at  low  tide  half 


FIG.    46.  —  Under  surface  of  Starfish    (Gom'aster  reticulatus) ,   showing   ambulacral 
grooves  and  protruded  suckers. 


buried  in  the  sand,  or  slowly  gliding  over  the  rocks. 
Cold  fresh  water  quickly  kills  them.  They  have  to  a 
high  degree  the  power  of  casting  off  their  rays  and  of 
reproducing  the  lost  parts.  They  are  carnivorous,  very 
voracious,,  and  are  the  worst  enemies  of  the  oyster. 


90       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

About  two  hundred  and  fifty  species  are  known.  The 
common  starfish  (Asterias)  has  four  rows  of  feet  in  each 
ray.  Solaster,  the  "sun  star,"  has  numerous  rays  with 
two  rows  of  feet  in  each.  Goniaster  is  somewhat  pen- 
tagonal in  form  with  feet  arranged  as  in  the  "  sun  star." 
Asteroidea  are  found  as  fossils. 

CLASS  2.  —  Ophiuroidea 

These  are  star-shaped  echinoderms  with  a  central 
disk  and  five  flexible,  jointed  arms  distinctly  marked  off 


FIG.  47.  —  Ophiocoma  russet,  an  Ophiuran;  natural  size.     West  Indies. 


from  the  disk,  the  latter  containing  all  the  visceral  parts. 
There  is  no  anal  opening  and  the  madreporite  is  on  the 


ECHINODERMATA 


same  side  as  the  mouth.  Ambulacral  grooves  are  lack- 
ing and  the  tube  feet  are  rudimentary,  locomotion  being 
effected  by  movements  of  the  very  flexible  and  muscular 
arms. 

The  brittle  Sfor^Qfikiurd)  is  common  along  the  Atlan- 
tic coast.  Astrophyton,  the  "  basket  fish,"  has  rays  which 
are  very  much  branched. 

CLASS  3.  —  Echinoidea 

These  are  free  echinoderms  with  a  globular  or  disk- 
shaped  shell  composed  of  closely-joined  calcareous  plates. 
There  are  no  ambulacral  grooves,  the  tube  feet  projecting 
through  openings  in  the  plates  arranged  either  along 
meridional  lines  or  in  the  form  of  a  star-shaped  rosette. 

The  sea  urchin  is  encased  in  a  thin,  hollow,  spherical 
shell  covered  with  spines.15  The  mouth  is  underneath, 
and  contains  a 
dental  apparatus 
more  complicated 
than  that  of  any 
other  creature.  It 
leads  to  a  diges- 
tive tube,  which 
extends  spirally 
to  the  summit  of 
the  body.  The 
spines  are  for  bur- 
rowing and  loco- 
motion, and  are 
moved  by  small 

mncr>1^o     <=»       \\    K 
JSC  ieS,    C 

ing  articulated  by 

ball-and-socket  j'oint  to  a  distinct  tubercle.  When  stripped 
of  its  spines,  the  shell  (or  "test")  is  seen  to  be  formed 
of  a  multitude  of  pentagonal  plates,  fitted  together  like  a 


4^'  ~  Under  surface  of  a  Sea  Urchin  (Echinus  escu- 
lentus)  ,  showing  the  mouth,  tips  of  the  teeth,  and  rows 
of  suckers  among  the  spines.  British  seas. 


92       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

mosaic.16  Five  double  rows  of  plates,  passing  from  pole 
to  pole,  like  the  ribs  of  a  melon,  alternate  with  five  other 
double  rows.  In  one  set,  called  the  ambulacra,  the  plates 
are  perforated  for  the  protrusion  of  tubular  feet,  or 
suckers,  as  in  the  starfish.  So  that  altogether  there  are 
twenty  series  of  plates — ten  ambulacral,  and  ten  inter- 
ambulacral.  The  shell  is  not  cast,  but  grows  by  the 
enlargement  of  each  individual  plate,  and  the  addition 
of  new  ones  around  the  mouth  and  the  opposite  pole. 
Echini  live  near  the  shore,  in  rocky  holes  or  under  sea- 
weed. They  are  less  active  than  the  starfishes  ;  and  feed 
almost  entirely  upon  seaweed.  They  reproduce  by  eggs. 
Regular  Echini,-as  the  common  Arbacia  or  purple  sea 
urchin,  and  the  green  sea  urchin,  are  nearly  globular, 
and  the  oral  and  anal  openings  are  opposite.  Irregular 
Echini,  as  the  Clypeaster,  are  flat,  and  the  anal  orifice  is 
near  the  margin,  as  in  the  "sand  dollar"  or  "cake 
urchin  "  (Echinarachnius). 

CLASS  4.  —  Holothuroidea 

These  wormlike  "  sea  slugs,"  as  they  are  called,  have 
a  soft,  elongated  body,  with  a  tough,  contractile  skin 


FIG.  49.    -Sea  Slugs  {Holothitria). 


containing  small  calcareous  plates.     One  end  is  abruptly 
terminated,  and  has  a  simple  aperture  for  a  mouth,  en- 


ECHINODERMATA  93 

circled  with  feathery  tentacles.  There  are  usually  five 
longitudinal  rows  of  ambulacral  suckers,  but  only  three 
are  used  for  locomotion,  of  which  one  is  more  developed 
than  the  rest.  The  mouth  opens  into  a  pharnyx  lead- 
ing to  a  long  intestinal  canal  extending  through  the 
body.  Holothurians  have  the  singular  power  of  eject- 
ing most  of  their  internal  organs,  surviving  for  some 
time  the  loss  of  these  essential  parts,  and  afterward 
reproducing  them.  They  occur  on  nearly  every  coast, 
especially  in  tropical  waters,  where  they  sometimes 
attain  the  length  of  three  or  four  feet.  As  found  dn  the 
beach  after  a  storm,  or  when  the  tide  is  out,  they  are 
leathery  lumps,  of  a  reddish,  brownish,  or  yellowish 
color.  They  may  be  likened  to  a  sea  urchin  devoid  of  a 
shell,  and  long  drawn  out,  with  the  axis  horizontal, 
instead  of  vertical.  They  feed  on  small  animals  which 
they  catch  with  their  tentacles,  and  upon  organic  par- 
ticles from  the  sand. 

CLASS  5.  —  Crinoidea 

The  crinoids,  or. "sea  lilies,"  are  fixed  to  the  sea 
bottom,  temporarily  or  permanently,  by  means  of  a  hol- 
low, jointed,  flexible  stem.  On  the  top  of  the  stem  is 
the  body  proper,  resembling  a  bud  or  expanded  flower, 
containing  the  digestive  apparatus,  and  bearing  the 
branched  arms.  The  mouth  looks  upward.  There  is  a 
complete  skeleton  for  strength  and  support,  the  entire 
animal  —  body,  arms,  and  stem  —  consisting  of  thou- 
sands of  pieces  embedded  in  the  tissue  of  the  body. 
Crinoids  were  very  abundant  in  the  old  geologic  seas, 
and  many  limestone  strata  were  formed  out  of  their 
remains.  They  are  now  nearly  extinct :  dredging  in  the 
deep  parts  of  the  oceans  has  brought  to  light  a  few  living 
representatives.  Pentacrinus  is  permanently  attached, 
but  the  rosy,  or  feather  star,  is  free  during  its  adult  life. 


94       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


FIG.  50.  —  A  living  Crinoid  (Pentacrinus  asteria)  one  fourth  natural  size. 
West  Indian  Seas. 


ANNULATA 


95 


Branch  IX.  —  ANNULATA 

The  Annelidas  include  the  highest  and  most  special- 
ized Worms.  They  have  many  segments,  spines,  or 
suckers  for  locomotion,  a  supra-esophageal  brain,  a 
ventral  chain  of  ganglia,  and  usually  a  closed  blood  sys- 
tem. 

There  are  two  principal  classes  :  Chcetopoda,  or  bristle- 
footed  worms ;  and  Hirudinea,  or  leeches.  The  former 


FIG.  51.  —  Marine  Worm  (Cirratulus  grandis},  with  extended  cirri.     Atlantic. 

class  includes  the  earthworms  {Liimbricus  and  Allolobo- 
phora),  the  sandworm  (Nereis),  and  the  lobworm  (Areni- 
cola). 

The  earthworm  develops  from  eggs  laid,  several  in  a 
capsule,  in  the  earth  or  near  refuse  heaps,  under  boards 


Q6        STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


and  straw.  '  Its  cylindrical  body  consists  of  numerous 
segments,  the  wall  being  very  muscular  and  covered 
by  a  tough,  smooth,  transparent  cuticle.  The  body 
cavity  is  subdivided  by  numerous  transverse  membra- 
nous partitions.  The  digestive  system  extends  through- 
out the  body  and  there  are  well-developed  nervous  and 
circulatory  systems.  The  former  lies 
mainly  below  the  digestive  organs  and 
consists  of  a  nerve  cord  running  the 
length  of  the  body.  This  cord  is 
composed  of  pairs  of  ganglia  con- 
nected by  longitudinal  and  transverse 
branches.  Above  the  mouth  opening 
is  a  pair  of  ganglia  forming  the 
"  brain."  The  larger  blood  vessels 
surround  the  esophagus  and  one,  the 
dorsal  vein,  lies  above  the  digestive 
system  and  may  be  seen  through  the 
FIG.  52.  -  Earthworm,  in-  skin  on  the  back.  There  are  no  well- 

ternal     anatomy    of    the       ir-i  tii«i 

anterior  region.  The  body  defined  sense  organs,  although,  judg- 
has  been  opened  along  the  mg  from  experiments,  the  earthworm 

dorsal  line  and  the  inter- 
nal organs  turned  to  the  is  sensitive    to  touch,  is  affected  by 

left;    ph,     pharynx;     h,  ...  .  ,. 

"heart";   /,   muscular  changes  in  the  intensity  of  light,  and 
partitions;   «,,  waii  of  ^^  evidence  of  having  the  sense  of 


body;  r,  reproductive  or- 
gans (in  part) ;  dv,  dorsal 
blood  vessel ;  i,  intestine ; 
g,  "  brain  "  ;  vn,  ventral 
nerve  cord;  si,  subintes- 
tinal  blood  vessel;  sn, 
subneural  blood  vessel. 


taste.  Respiration  is  carried  on  by 
the  vascular  skin,  there  being  no 
lungs  or  gills.  Earthworms  feed 
upon  decaying  vegetable  matter  and 
upon  organic  particles  contained  in  the  earth,  swallowed 
in  the  process  of  making  the  burrow,  or  for  the  sake  of 
the  contained  food.  The  refuse  from  the  body  is  piled 
up  around  the  mouth  of  the  burrow  in  the  form  of  pel- 
lets. The  amount  of  earth  annually  brought  up  from 
the  deeper  layers  of  the  soil  is  sufficient  to  be  of  consid- 
erable geological  and  economic  importance. 


ARTHROPODA  97 

The  earthworm  belongs  to  the  subclass  Oligochceta, 
the  members  of  which  have  but  few  bristles  on  each 
segment. 

Nereis  lives  in  the  sea,  under  rocks  and  among  sea- 
weeds. Like  the  earthworm,  it  has  a  distinctly  seg- 
mented body.  There  is  a  well  denned  head,  bearing 
sense  organs,  as  eyes  and  tentacles.  The  throat  is 
provided  with  two  protrusible  jaws,  by  means  of  which 
the  worm  seizes  its  food,  often  living  prey  (Fig.  215). 
Each  segment  bears  a  pair  qf  flattened,  paddlelike  par- 
apodia,  which  enable  the  worm  to  swim  rapidly.  The 
arrangement  of  the  digestive,  nervous,  and  circulatory 
systems  is  much  like  that  seen  in  the  earthworm. 

Nereis  is  a  member  of  the  subclass  Polychceta,  which 
is  characterized  by  the  presence  of  numerous  bristles  on 
each  segment. 

The  leeches  are  externally  segmented,  usually  flat- 
tened, and  have  a  sucking  disk  at  each  end  of  the  body. 
The  mouth  is  in  the  anterior  disk  and  is  provided  with 
three  semi-circular,  saw-toothed  jaws,  by  means  of  which 
the  leech  makes  the  incision  through  which  it  sucks  the 
blood  of  its  prey.  The  disks  are  also  used  for  locomo- 
tion. The  digestive  system  is  very  capacious,  and  some 
leeches  can  live  even  if  not  fed  more  often  than  once  in 
two  or  three  months.  Leeches  are  generally  fresh- 
water animals,  though  some  kinds  are  found  in  the  sea 
and  others  live  on  land. 

Branch  X.  —  ARTHROPODA 

This  is  larger  than  all  the  other  branches  put  together, 
as  it  includes  the  animals  with  jointed  legs,  such  as 
crabs  and  insects.  These  differ  widely  from  the  mol- 
luscan  type  in  having  numerous  segments,  and  in  show- 
ing a  repetition  of  similar  parts ;  and  from  the  worms 
DODGE'S  GEN.  ZOOL.  —  7 


98       STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

in  having  jointed  appendages  and  a  definite  number  of 
segments. 

The  skeleton  is  outside,  and  consists  of  articulated 
segments  or  rings.  The  limbs,  when  present,  are  like- 
wise jointed  and  hollow.  The  jaws  move  from  side  to 
side.  The  nervous  system  consists  mainly  of  a  double 
chain  of  ganglia,  running  along  the  ventral  surface  of 
d  the  body  under  the 

alimentary  canal. 
The  brain  is  con- 
nected to  the  ventral 

FIG.  53.  —  Diagram  of  the  structure  of  an  Arthropod  o-Qncrlip      hv      p  rincr 

(after  Schmeil) :  a,  antenna  ;  c,  circulatory  system;  o""^o^  ""      ®J       **  '  ***§ 

d,  alimentary  canal;   n,  nerve  cord;  g,  ganglion;  encircling"     the  °"ul- 
s,  skeleton. 

let.    The  alimentary 

canal  and  the  circulatory  apparatus  are  nearly  straight 
tubes  lying  lengthwise — the  one  through  the  center, 
and  the  other  along  the  back.  The  skeleton  is  com- 
posed of  a  horny  substance  (chitin),  or  of  this  substance 
with  carbonate  of  lime.  All  the  muscles  are  nearly 
always  striated. 

There  are  five  classes,  of  which  the  first  almost  exclu- 
sively is  water  breathing,  having  gills,  and  the  others 
principally  air  breathing,  being  provided  with  tracheae. 

CLASS  i. — Crustacea 

The  Crustacea,  with  few  exceptions,  are  water  breath- 
ing Arthropoda,  usually  with  two  pairs  of  antennae.17 
Among  them  are  the  largest,  strongest,  and  most  vora- 
cious of  the  branch,  armed  with  powerful  claws  and 
a  hard  cuirass,  bristling  with  spines.  Although  con- 
structed on  a  common  type,  crustaceans  exhibit  a  won- 
derful diversity  of  external  form :  contrast,  for  example, 
a  barnacle  and  a  crab.  We  will  select  the  lobster  as 
illustrative  of  the  entire  group. 


ARTHROPODA 


99 


A  typical  crustacean  consists  of  twenty  segments,  of 
which  five  belong  to  the  head,  eight  to  the  thorax,  and 
seven  to  the  abdomen.18  In  the  lobster,  however,  as  in 
all  the  higher  forms,  the  joints  of  the  head  and  thorax 
are  welded  together  into  a  single  piece,  called  the 
c^pkato thorax.  On  the  front  of  this  shield  is  a  pointed 
process  or  rostrum;  and  attached  to  the  last  joint  of 
the  abdomen  (the  so-called  "  tail  ")  is  the  sole  repre- 
sentative of  a  tail 
—  the  telson.  The 
skeleton  is  a  mix- 
ture of  chitin  and 
calcareous  matter.19 
On  the  under  side 
of  the  body  we  find 
numerous  appen- 
dages, feelers,  jaws, 
claws,  and  legs  be- 
neath the  cephalo- 
thorax,  and  flat 
swimmerets  under 
the  abdomen.  In 
fact,  every  segment 
except  the  last,  car- 
ries a  pair  of  mov- 
able appendages, 
consisting  typically 

Of    a     Stalk    Or   protO-  FIG.  54.  -  Under  side  of  the  Crayfish,  or  Fresh-water 

.     '  Lobster  (Astacus  ftnviatilis} :  a,  first  pair  of  an- 

Podlte,    bearing    tWO  tennse;    b,  second  pair;    c,  eyes;    d,  opening  of 

v              i            ,1  kidney;    e,  foot  jaws;  f,  g,  first  and  fifth  pair  of 

branches,  the    eXOpO-  thoracic  legs;  A,  swimmerets;   i,  anus;  k,  caudal 

dite  and  the  endopo-       fin- 

dite.  The  five  segments  of  the  head  are  compressed 
into  a  very  small  space,  yet  have  the  following  mem- 
bers :  2°  the  short  and  the  long  antennae ;  the  mandibles, 
or  jaws,  between  which  the  mouth  opens;  and  the 


k 


100     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

two  pairs  of  maxillae.  The  thorax  carries  three  pairs 
of  modified  limbs,  called  "foot  jaws,"  and  five  pairs 
of  legs.  The  foremost  legs,  "the  great  claws,"  are 
extraordinarily  developed,  and  terminated  by  strong 
pincers  (chelce).  Of  the  four  slender  pairs  succeeding, 
two  are  furnished  with  claws,  and  two  are  pointed.  The 
last  pair  of  swimmerets,  together  with  the  telson,  form 
the  caudal  fin  —  the  main  instrument  of  locomotion ; 
the  others  (called  "  swimmerets ")  are  used  by  the 
female  for  carrying  her  eggs.  The  eyes  are  raised  on 
stalks,  so  as  to  be  movable  (since  the  head  is  fixed  to 
the  thorax),  and  are  compound,  made  up  of  about  two 


FIG.  55. —  Internal  anatomy  of  the  Crayfish:  em,  extensor  muscle  of  abdomen;  Jm, 
flexor  muscle  of  abdomen;  m,  mouth;  cs,  cardiac  portion  of  stomach;  ps,  pyloric 
portion  of  stomach;  dg,  digestive  gland;  kt  heart;  r,  reproductive  gland;  i,  intes- 
tine; g,  "  brain  "  ;  vc,  ventral  nerve  cord. 


thousand  five  hundred  square  facets.  On  the  base  of 
each  small  antenna  is  a  minute  sac,  whose  mouth  is 
guarded  by  hairs :  this  is  the  organ  of  hearing.  The 
gills,  twenty  on  a  side,  are  situated  at  the  bases  of  the 
legs  and  inclosed  in  two  chambers,  into  which  water  is 
freely  admitted,  in  fact,  drawn,  by  means  of  a  curious 
attachment  to  one  of  the  maxillae,  which  works  like 
a  paddle  or  scoop.  The  heart  is  a  single  oval  cavity, 
and  drives  arterial  blood  —  a  milky  fluid  full  of  corpus- 
cles. The  alimentary  canal  consists  of  a  short  gullet, 


ARTHROPGbA,    ;     :  ;••;     .";,-, 

a  gizzardlike  stomach  containing  teeth,  and  a  straight 
intestine. 

Crustaceans  pass  through  a  series  of  strange  metamor- 
phoses before  reaching  their  adult  form.  They  also 
periodically  cast  the  shell,  or  molt,  every  part  of  the 
integument  together  with  the  lining  of  the  gullet  and 
stomach  being  renewed  ;  and  another  remarkable  endow- 
ment is  the  spontaneous  rejection  of  limbs  and  their 
complete-  'restoration.  Many  species  are  found  in 
fresh  water,  but  the  class  is  essentially  marine  and 
carnivorous. 

Of  the  numerous  orders  of  this  great  class  we  will 
mention  only  the  following  :  — 

1 .  Phyllopoda ;   small,    almost    microscopic,    aquatic 
Crustacea,    with   the    appendages    showing    very   little 

differentiation,  no  gastric  teeth,  the 
body  distinctly  segmented  and  cov- 
ered by  a  cephalic  shield.  The 
a  appendages  posterior  to  the  head 
are  leaflike,  hence  the  name  of  the 

FIG.    56.  -  Water   Fleas:    a, 

Cyclops  (after  Vosseier) ;   order.     Included  here  are  the  brine 

b,    Daphnia     (after     Vos-        .      .  f    .  .    N  '     . 

seier);   c,  Cypris   (after  shrimp    (Artemia),    and    the   fresh- 
water forms  Branchipus  and  Daph- 
nia, the  bivalve  shell  of  the  latter  giving  it  the  appear- 
ance of  a  mollusk  (Fig.  56). 

2.  Ostracoda;  minute  Crustacea  with  an  unsegmented 
body   inclosed   in  a   bivalve   carapace   or   shell.     This 
order  is  represented  in  fresh  water  by  Cypris  (Fig.  56). 

3.  Copepoda  ; .  mostly  of  small  size,  with  an  elongated 
and,  usually,  a  distinctly  segmented  body  without  dorsal 
shell.     In  this  order  belong  the  fish  lice,  and  the  water 
flea  (Cyclops}  of  fresh  water,   the  female  of  which   is 
often  seen  darting  about  in  aquarium  jars  bearing  its 
two  egg  masses  attached  to  the  abdomen  (Fig.  56). 

4.  Cirripedia ;  marine    Crustacea,     imperfectly    seg- 


2:  .STWGTURA-L   AND   SYSTEMATIC   ZOOLOGY 


mented,  and  fixed  or  parasitic   in    adult   life,    growing 
head  downward  in  their  shell.     The  feathery,  branched, 


FlG.  57.  —  Barnacles,  or  Pedunculate  Cirripedes  (Lepas  anatijera). 

thoracic  feet  are  protruded  through  the  opening  of  the 
shell  to  grasp  particles  of  food.  Lepas,  the  ship 
barnacle,  grows  attached  to  floating  timber  and  the 
bottoms  of  ships  by  a  long,  leathery 
stalk  (Fig.  57).  The  acorn  shells 
(Balamis}  grow  on  rocks  between 
tide  marks,  their  white,  conical 
"  shells "  forming  an  incrusting 
layer  on  the  rock  (Fig.  58). 

5.  Decapoda ;  large,  highly  or- 
ganized crustaceans,  having  usually 
a  thorax  of  eight  and  an  abdomen 
of  seven  segments,  the  anterior  re- 
gions of  the  carapace  united  to  form  a  cephalothorax ; 
the  eyes  are  borne  on  stalks  and  the  gills  are  thoracic. 


FIG.  58.  — Acorn  Shells  (Ba- 
lanus}  on  the  shell  of  a  whelk 
(Bucctnum), 


ARTHROPODA 


103 


There  are  ten  legs.     Here  belong  the  lobster  (Homarus) 
(Fig.  59),  and  "crayfishes  (Astacus  and  Cambarus)  (Fig. 


FIG.  59.  —  Lobster  (Homarus  -vulgaris) 


FIG.  60.  —  Swimming  Crab  (Platyonychus). 

54),   prawn  (Palczmon),  shrimp  (Crangori),  hermit  crab 
(Pagurus),  and  crab  (Platyonychus)  (Fig.  60). 


IO4       STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 


Crabs  differ  from  lobsters  chiefly  in  being  formed 
for  creeping  at  the  bottom  of  the  sea  instead  of  swim- 
ming, and  in  the  reduction  of  the 
abdomen  or  "  tail  "  to  a  rudiment, 
which  folds  into  a  groove  under 
the  enormous  thorax.  They  are 
the  highest  and  largest  of  living 
Crustacea  :  they  have  been  found 
at  Japan  measuring  twenty  feet 
between  the  tips  of  the  claws. 

6.  Arthrostraca  ;  with  the  tho- 
rax reduced  to  six  or  seven  seg- 
ments owing  to  the  fusion  of  one 
or  two  of  the  anterior  thoracic 
segments  with  the  head ;  eyes 
usually  sessile.  This  order  in- 
eludes  the  wood  louse  or  sow 

U.  S.  coast.  . 

bug    (Omsciis)    found    in     damp 

places,   the    slaters   (Idotea)  (Fig.    61),    and   the    sand 
fleas  (Gammarus). 


FIG.  62.  —  Antpkithoe  maculata  :  a  sand  flea. 

CLASS  2.  —  Onycophora 

This  class  includes  only  a  single  genus,  Peripatus, 
the  species  of  which  are  all  terrestrial,  living  in  damp 
places,  and  are  confined  mainly  to  the  Southern  Hemi- 
sphere. Peripatus  is  a  cylindrical,  soft-bodied  animal 
resembling  a  caterpillar,  though  the  body  is  not  seg- 
mented. There  is  a  plainly  marked  head  bearing  a 
pair  each  of  eyes,  antennae,  and  jaws.  The  body  is 


ARTHROPODA  105 

supported  on  many  pairs  (fourteen  to  forty-two,  accord- 
ing to  the  species)  of  short,  fleshy  appendages  which 
are  not  jointed.  These  animals  are  chiefly  of  interest 
because  of  the  fact  that  in  certain  features  of  structure, 
as  the  size  of  the  brain,  the  presence  of  tracheae,  the 


FIG.  63.  —  Perifatus  ;  natural  size. 

arrangement  of  the  circulatory  system,  and  the  clawed 
appendages,  and  in  their  mode  of  development,  they 
resemble  the  Arthropods,  while  in  other  respects,  espe- 
cially as  regards  the  excretory  and  nervous  systems 
they  approach  the  Annulata  and  the  Flatworms,  Thus, 
the  class  serves  to  connect  the  Arthropods  and  the 
"  Worms." 

CLASS  3.  —  Myriapoda 

Myriapods  are  air-breathing  Arthropods  having  the 
body  divided  into  similar  segments,  so  that  thorax  and 
abdomen  are  scarcely  distinguishable.  They  resemble 
worms  in  form  and  in  the  simplicity  of  their  nervous 
and  circulatory  systems ;  but  the  skin  is  stiffened  with 
chitin,  and  the  legs  (indefinite  in  number)  are  articu- 
lated. The  legs  resemble  those  of  insects,  and  the 
head  appendages  follow  each  other  in  the  same  order 
as  in  insects  —  eyes,  antennae,  mandibles,  maxillae,  and 
labium.  They  breathe  by  tracheae,  and  have  two 
antennae  and  a  pair  of  eyes. 

There  are  two  important  orders  :  — 

i.  Chilopoda,  characterized  by  haying  a  flattened  body 
composed  of  about  twenty  segments,  each  carrying  one 
pair  of  legs,  of  which  the  hindermost  is  converted  into 


106      STRUCTURAL  AND    SYSTEMATIC  ZOOLOGY 

spines.  They  have  longer  antennae  than  the  preceding, 
and  the  mouth  is  armed  with  two  formidable  fangs  con- 
nected with  poison  glands.  They*are  carnivorous  and 
active.  Such  is  the  Centipede  (Scolopendra,  Fig.  80). 

2.  Diplopoda,  having  a  cylindrical  body,  each  segment, 
except  the  anterior,  being  furnished  with  two  pairs  of 
legs.  They  are  slow  of  locomotion,  harmless,  and  vege- 
tarian. The  thousand-legged  worm  (Julus)  is  a  common 
representative. 

CLASS  4.  —  Insecta 

Insects  are  distinguished  by  having  head,  thorax,  and 
abdomen  distinct,  three  pairs  of  jointed  legs,  one  pair  of 
antennae,  and  generally  two  pairs  of  wings.  The  number 
of  segments  in  the  body  never  exceeds  twenty.  The 
head,  apparently  one,  is  formed  by  the  union  of  four 
segments.  The  thorax  consists  of  three,  — the  prothorax, 
mesothorax,  and  metathorax,  —  each  bearing  a  pair  of 
legs;  the  wings,  if  present,  are  carried  by  the  last  two 
segments  (Fig.  295).  The  abdomen  is  usually  composed 
of  ten  segments,  more  or  less  movable  upon  one  another. 
The  skin  is  hardened  with  chitin,  and  to  it,  as  in  all 
Arthropods,  the  muscles  are  attached.  All  the  append- 
ages are  hollow. 

The  antennae  are  inserted  between  or  in  front  of  the 
eyes.  There  is  a  great  variety  of  forms,  but  all  are 
tubular  and  jointed.  They  are  supposed  to  be  organs 
of  touch,  and  seem  also  to  be  sensitive  to  sound  and  odor 
(Fig.  344).  The  eyes  are  usually  compound,  composed 
of  a  large  number  of  hexagonal  corneae,  or  facets  (from 
fifty  in  the  ant  to  many  thousands  in  the  winged  insects) 
(Fig.  352).  They  are  never  placed  on  movable  stalks,  as 
the  lobster's.  Besides  these,  there  are  three  simple  eyes, 
called  ocelli.  The  mouth  may  be  fitted  for  biting  (masti- 
catory), as  in  beetles,  or  for  sucking  (suctorial),  as  in 


ARTHROPODA 


ID/ 


butterflies.     The   masticatory  type,  which  is  the  more 
complete,     and     of 
which  the  other  is  ~ 

but  a  modification, 
consists  of  four 
horny  jaws  (mandi- 
bles and  maxilla) 
and  an  upper  and 
an  under  lip  (lab mm 
and  labium).  Sensi- 
tive palpi  (maxillary 
and  labial}  are  de- 
veloped from  the 
lower  jaw  and  lower 
lip.  The  labium  is 
also  prolonged  into 
a  ligula,  or  tongue 
(Figs.  219,220,221). 
The  legs  are  in- 
variably six  in  the 
adult,  the  fore  legs 
directed  forward 
and  the  hinder  pairs 
backward.  Each 
consists  of  a  hip, 
thigh,  shank,  and 
foot.21  Some  larvae 
have  also  "  false 
legs,"  without  joints, 
on  the  abdomen, 
upon  which  they 
chiefly  rely  in  loco- 
motion (Fig.  73). 
The  wings  are  ex- 


—  Under  surface  of  a  Beetle  (Harpalus  caligi- 
nosus)'.  a,  ligula;  b,  paraglossae;  c,  supports  of 
labial  palpi;  d,  labial  palpus;  ef  mentum;/",  inner 
lobe  of  maxilla;  .g,  outer  lobe;  h,  maxillary  palpus; 
/,  mandible;  k,  buccal  opening;  /,  gula,  or  throat; 
MI,  buccal  sutures;  «,  gular  suture;  o,  prosternum; 
/,  episternum  of  prothorax;  /',  epimeron;  q,  g'  ,  g", 
coxae;  r,  r,  r,  trochanters;  s,  s'  ,  s",  femora,  or 
thighs;  t,  t,'  t",  tibiae;  v,  ventral  abdominal  seg- 
ments; w,  episterna  of  mesothorax;  x,  mesoster- 
num;  y,  episterna  of  metathorax;  y',  epimeron;  z, 
metasternum. 


pansions  of  the  crust,  stretched  over  a  network  of  horny 


108     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

tubes  (Fig.  278).  The  venation,  or  arrangement  of 
these  tubes  (called  veins  and  veinlets),  particularly  in 
the  fore  wings,  is  peculiar  in  each  genus.  In  many 
insects,  the  abdomen  of  the  female  ends  in  a  tube  which 
is  the  sheath  of  a  sting,  as  in  .the  bee,  or  of  an  ovipositor, 
or  "  borer,"  as  in  the  ichneumon,  by  means  of  which  the 
eggs  are  deposited  in  suitable  places. 

Cephalization  is  carried  to  its  maximum  in  this  class, 
and  we  have  animals  of  the  highest  instincts  under  the 
articulate  type.  The  "brain  "  is  formed  of  several  gan- 
glia massed  together,  and  lies  across  the  upper  side 
of  the  throat,  just  above  the  mouth.  The  main  nerve 
cord  lies  along  the  ventral  side  of  the  body,  and  bears 
several  large  ganglia ;  besides  this,  there  is  a  visceral 
nerve  representing,  in  function,  the  sympathetic  system 
of  vertebrates.  The  digestive  apparatus  consists  of  a 
pharynx,  gullet  (to  which  a  crop  is  added  in  the  fly, 
butterfly,  and  bee  tribes),  gizzard,  stomach,  and  intestine 
(Figs.  239,  240,  241).  There  are  no  absorbent  vessels, 
the  chyme  simply  transuding  through  the  walls  of  the 
canal.  The  blood,  usually  a  colorless  liquid,  is  driven 
by  a  chain  of  hearts  along  the  back,  i.e.,  by  a  pulsating 
tube  divided  into  valvular  sacs,  ordinarily  eight,  which 
allow  the  current  to  flow  only  toward  the  head.  As  it 
leaves  this  main  pipe,  it  escapes  into  the  cavities  of  the 
body,  and  thus  bathes  all  the  organs.  Although  the 
blood  does  not  circulate  in  a  closed  system  of  blood 
vessels,  as  in  vertebrates,  yet  it  always  takes  one  set  of 
channels  in  going  from  the  heart,  and  another  in  return- 
ing. Respiration  is  carried  on  by  tracheae,  a  system  of 
tubes  opening  at  the  surface  by  a  row  of  apertures 
{spiracles),  generally  nine  on  each  side  of  the  body  (Figs. 
276,  277,  278). 

The  sexes  are  distinct,  and  the  larvae  are  hatched  from 
eggs.  As  a  rule,  an  insect,  after  reaching  the  adult,  or 


ARTHROPOD A 


109 


imago  state,  lives  from  a  few  hours  to  several  years,  and 
dies  after  the  process  of  reproduction.  Growth  takes 
place  only  during  larval  life;-  and  all  metamorphoses 
occur  then.  Among  the  social  tribes,  as  bees  and  ants, 
the  majority  (called  "workers  ")  do  not  develop  either  sex. 

Insects  (the  six-footed  arthropods)  comprise  about 
one  half  of  the  whole  animal  kingdom  as  known,  more 
than  two  hundred  and  fifty  thousand  species  having  been 
described.  They  may  be  grouped  into  seven  principal 
orders : — 

i.  Orthoptera  have  four  wings :  the  front  pair  some- 
what thickened,  narrow,  and  overlapping  along  the  back ; 


FIG.  65.  —  Metamorphosis  of  a  Cricket  (Gryllus). 

the  hind  pair  broad,  net  veined,  and  folding  up  like  a  fan 
upon  the  abdomen.  The  hind  legs  are  usually  large,  and 
fitted  for  leaping,  all  the  species  being  terrestrial,  although 
some  fly  as  well  as  leap.  The  eyes  are  small,  the  mouth 
remarkably  developed  for  cutting  and  grinding.  The  lar- 


1 10     STRUCTURAL   AND  SYSTEMATIC   ZOOLOGY 

vae  and  pupae  are  active  and  resemble  the  imago.  They 
are  nearly  all  vegetarian.  Each  family  produces  char- 
acteristic sounds  (stridulation).  About  ten  thousand 
species  have  been  described.  The  representative  forms 
are  crickets  (Gryllus),  locusts  (Melanoplus\  grasshoppers 
(Orchelimum),  walking  sticks  {Diapheromera\  and  cock- 
roaches {Periplaneta). 

2.    Neuroptera  have  a  comparatively  long,  slender  body, 
and  four  large,  transparent  wings,  nearly  equal  in  size, 


FIG.  66. —  Metamorphosis  of  an  Hemipter,  Water  Boatman  (Notonecta). 

membranous  and  lacelike.  The  mouth  parts  are  adapted 
for  biting.  Among  them  are  the  brilliant  dragon  flies, 
or  devil's  darning  needles  (Libelhila),  well  known  by  the 
enormous  head  and  thorax,  large,  prominent  eyes  (each 
furnished  with  twenty-eight  thousand  polished  lenses), 
and  scorpionlike  abdomen ;  the  delicate  and  short-lived 
May  flies  {Ephemera) ;  caddis  flies  (Pkryganea),  whose 
larvae  live  in  a  tubular  case  made  of  minute  stones, 
shells,  or  bits  of  wood  ;  the  horned  corydalis  (C&rydalis^ 
of  which  the  male  has  formidable  mandibles  twice  as 
long  as  the  head;  and  the  white  ants  (Termes)  of  the 
tropics. 


ARTHROPODA 


III 


FIG.  67.  —  Seventeen-year  Cicada  (Cicada  septendecim):  a,  pupa;  b,  the  same,  after 
the  imago,  c,  has  escaped  through  a  rent  in  the  back;  d,  holes  in  a  twig,  where  the 
eggs,  e,  are  inserted. 


FIG.  68.  —  Dragon  Fly  (Ltiellula). 


112     STRUCTURAL  AND    SYSTEMATIC    ZOOLOGY 

3.  Hemiptera,  or  "bugs,"  are  chiefly  characterized  by 
a  suctorial  mouth,  which  is  produced  into  a  long,  hard 
beak,  in  which  mandibles  and  maxillae  are  modified  into 
bristles  and  inclosed  by  the  labium.     The  four  wings  are 
irregularly  and  sparsely  veined,  sometimes  wanting.    The 
body  is  flat  above,  and  the  legs  slender.     The  larva  differs 
from  the  imago  in  wanting  wings.     In  some  species  the 
fore  wings  are  opaque  at  the  base  and  transparent  at  the 
apex,  whence  the  name  of  the  order.     Some  feed  on 
the  juices  of  animals,  others  on  plants.     Here  belong  the 
wingless  bed  bug  (Cimex)  and  louse  (Pediculus),  the 
squash  bug  (Anasd),  water  boatman  (Notonectd),  seven- 
teen-year locust  (Cicada),  cochineal  (Coccus),  and  plant 
louse  (Aphis).     More  than  twenty  thousand  species  are 
known. 

4.  Dipt  era,  or  "  flies,"  are  characterized  by  the  rudi- 
mentary state  of  the  hinder  pair  of  wings.     Although 
having,  therefore,  but  one  available  pair,  they  are  gifted 
with  the  power  of  very  rapid  flight.     While  a  bee  moves 
its  wings  one  hundred  and  ninety  times  a  second,  and  a 


FIG.  69.  —  Metamorphosis  of  the  Flesh  Fly  (Sarcophaga  carnaria) :  a,  eggs;  b,  young 
maggots  just  hatched:  c,  d,  full-grown  maggots;  e,  pupa;  f,  imago. 

butterfly  nine  times,  the  house  fly  makes  three  hundred 
and  thirty  strokes.  A  few  species  are  wingless.  The 
eyes  are  large,  with  numerous  facets.  In  some  forms,  as 
the  house  fly,  all  the  mouth  parts,  except  the  labium,  are 
rudimentary ;  and  the  labium  has  an  expanded  tip,  by 
means  of  which  the  fly  licks  up  its  food.  In  other  forms, 
as  the  mosquito,  the  other  mouth  parts  are  present  as 
bristles  or  lancets,  fitted  for  piercing;  the  thorax  is 
globular,  and  the  legs  slender.  The  larvae  are  footless 


ARTHROPODA  113 

grubs.  The  Diptera  number  about  forty  thousand. 
Among  them  are  the  mosquitoes  (Culex)\  Hessian  fly 
(Cecidomyid),  so  destructive  to  wheat;  daddy  longlegs 
or  crane  fly  (Tipula),  resembling  a  gigantic  mosquito; 
the  wingless  flea  (Pulex) ;  besides  the  immense  families 
represented  by  the  house  fly  (Mused)  and  bot  fly  (CEstrus). 
5.  Lepidoptera,  or  " butterflies "  and  "moths,"  are 
known  chiefly  by  their  four  large  wings,  which  are 
thickly  covered  on  both  sides  by  minute,  overlapping 
scales.  The  scales  are  of  different  colors,  and  are  often 
arranged  in  patterns  of  exquisite  beauty.  They  are  in 
reality  modified  hairs,  and  every  family  has  its  particular 


FIG.  70.  —  Scales  from  the  Wings  of  various  FIG.  71.  —  Part  of  the  Wing  of  a  Moth 

Lepidoptera.  (Santia),  magnified   to  show  the 

arrangement  of  scales. 

form  of  scale.  The  head  is  small,  and  the  body  cylin- 
drical. The  legs  are  of  but  little  use  for  locomotion. 
All  the  mouth  parts  are  nearly  obsolete  except  the  maxil- 
lae, which  are  fashioned  into  a  "  proboscis  "  for  pumping 
up  the  nectar  of  flowers.  The  larvae,  called  "cater- 
pillars," have  a  wormlike  form,  and  from  one  to  five 
pairs  of  abdominal  legs,  or  "  false  legs,",,  in  addition  to 
the  three  on  the  thorax.  The  mouth  is  formed  for  mas- 
tication, and  (except  in  the  larvae  of  butterflies)  the  lip 
has  a  spinneret  connected  with  silk  glands  (Fig.  75). 

There  are  two  groups :    the  gay  butterflies,  having 
knobbed  or  hooked  antennae,  and  flying  in  the  day  only, 
forming  one  group ;    and  the  moths,  which  generally 
DODGE'S  GEN.  ZOOL.  —  8 


114     STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 
prefer  the  night,  and  whose  antennae  are  threadlike  and 


FIG.  72. —  Vanessa  polychloros,  or  "  Tortoise-shell  Butterfly." 

often  feathery,  composing  the  second  group.     To  this 
belong  the    dull-colored    Sphinges   or    "  hawk   moths," 


FlG.  73.  — Moth  and  Larva  of  Attacus pavonia-major . 

which  have  antennae  thickened  in  the  middle,  and  which 


ARTHROPODA 


fly  at  twilight.     Generally,  when  at  rest,  the  butterflies 
keep  their  wings  raised  vertically,  while  the  others  hold 


FIG.  74.  —  Fruit  Moth  (Carpocapsa  pomonella)  :  b,  larva;  a,  chrysalis;  c,  imago. 

theirs  horizontally.  The  pupa  of  the  former  is  unpro- 
tected, and  is  usually  suspended  by  a  bit  of  silk  ;  the 
pupa  of  the  moths  is  in- 
closed in  a  cocoon. 

From  twenty-two  thou- 
sand to  twenty-five  thou- 
sand lepidopterous  species 
have  been  identified.  Some 
of  the  most  common  but- 
terflies are  the  swallow-tail 
Papilio,  the  white  Pieris, 
the  sulphur-yellow  Colias  ; 
the  Argynnis,  with  silver 
spots  on  the  under  side  of 
the  hind  wings;  the  Va- 
nessa, with  notched  wings. 

The    Sphinges   exhibit    little     FlG   75--  Head  of  a  Caterpillar,  from  be- 

0  neath:  a,  antennae;    b,  horny  Jiws;   c, 

Variety.  They     have 


thread  of  silk  from  the  conical  fusulus, 
,.    ,  ,          on  either  side  of  which  are  rudimentary 

row,    powerful   wings,    and      palpi.  Magnified. 


Il6     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


are  sometimes  mistaken  for  humming-birds.  The 
"potato  worm"  is  the  caterpillar  of  a  sphinx.  The 
most  conspicuous  moths  are  the  large  and  beautiful 
Telea,  distinguished  by  a  triangular,  transparent  spot 
in  the  center  of  the  wing ;  the  white  Bombyx,  or  "  silk- 
worm ; "  the  reddish-brown  Clisiocampa,  whose  larva, 
"  the  American  tent  caterpillar,"  spreads  its  web  in  many 
an  apple  and  cherry  tree  ;  the  pale,  delicate  Geometrids  ; 
and  the  small  but  destructive  Tineids,  represented  by 
the  clothes  moth. 

6.    Coleoptera,  or  "beetles."      This  is  the  largest  of 
the  orders,  the  species  numbering  about  ninety  thousand. 


FIG.  76.  —  a,  imago,  and  b,  larva,  of  the  Goldsmith  Beetle  (Cotalpa  lanigera) ;  c,  pupa 
of  June  Bug  (Lachnosterna/usca). 

They  are  easily  recognized  by  the  elytra,  or  thickened, 
horny  fore  wings,  which  are  not  used  for  flight,  but 
serve  to  cover  the  hind  pair.  When  in  repose,  these 
elytra  are  always  united  by  a  straight  edge  along  the 
whole  length.  The  hind  wings,  when  not  in  use,  are 
folded  transversely.  The  mandibles  are  well  developed, 
and  the  integument  generally  is  hard.  The  legs  are 
strong,  for  the  beetles  are  among  the  most  powerful 
running  insects.  The  larvae  are  wormlike,  and  the  pupa 
is  motionless.  The  highest  tribes  are  carnivorous.  The 
most  prominent  forms  are  the  savage  but  beautiful  tiger 


ARTHROPODA 


117 


beetles  (Cicindela)\  the  common  ground  beetles  (Har- 
palus\  whose  elytra  bear  parallel  ridges;  the  diving 
beetles  (Dytiscus),  with  boat-shaped  body,  and  hind 
legs  changed  into  oars;  the  carrion  beetles  (Silpha\ 


FIG.  77. —  Sexton  Beetles  (Necrophorus  vespillo),  with  larva  and  nymph.     They  are 
burying  a  mouse,  preparatory  to  laying  their  eggs  in  it. 

distinguished  by  their  black,  flat  bodies  and  club-shaped 
antennae ;  the  goliath  beetles  (Goliatkus),  the  giants  of 
the  order ;  the  click  beetles  (Alans) ;  the  lightning 
bugs  (Pyrophorus);  the  spotted  lady-birds  (Coccinella); 
the  showy,  long-horned  beetles  (Cerambycidce}\  and 


Il8     STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 

the    destructive   weevils  (furculionida),   with    pointed 
snouts. 


FIG.  78.  —  Metamorphosis  of  the  Mosquito  {Cnlex  pipiens). 

7.    Hymenoptera,  comprising  at  least  thirty-five  thou- 
sand species,  include  the  highest,  most  social,  and,  we 


ARTHROPODA  1 19 

may  add  (if  we  except  the  silkworm),  the  most  useful,  of 
insects.  They  have  a  large  head,  with  compound  eyes 
and  three  ocelli,  mouth  fitted  both  for  biting  and  lapping, 
legs  formed  for  locomotion  as  well  as  support,  and  four 
wings  equally  transparent,  and  interlocking  by  small 
hooks  during  flight.  The  females  are  usually  provided 
with  a  sting,  or  borer.  The  larvae  are  footless,  helpless 
grubs,  and  generally  nurtured  in  cells,  or  nests.  Such 
are  the  honey  bees  (Apis],  humble  bees  (Bombus\ 
wasps  (  Vespa\  ants  {Formica),  ichneumon  flies,  and  gall 
flies.  Those  living  in"  societies  exhibit  three  castes : 
females,  or  "  queens  "  ;  males,  or  "  drones  "  ;  and  neu- 
ters, or  sexless  "  workers."  •  There  is  but  one  queen  in  a 


a 

FIG.  79.  —  Honey  bee  (Apis  mellifica) :  a,  female;  b,  worker;  c,  male. 

hive,  and  she  is  treated  with  the  greatest  distinction,  even 
when  dead.  She  dwells  in  a  large,  pear-shaped  cell, 
opening  downward.  She  lays  three  kinds  of  eggs  :  from 
the  first  come  forth  workers,  the  second  produces  males, 
and  the  last  females.  The  drones,  of  which  there  are 
about  eight  hundred  in  an  ordinary  hive,  are  marked  by 
their  great  size,  their  large  eyes  meeting  on  the  top  of 
the  head,  and  by  being  stingless.  The  workers,  which 
number  twenty  to  one  drone,  are  small  and  active,  and 
provided  with  stings,  and  hollow  pits  on  the  thighs, 
called  "  baskets,"  in  which  they  carry  pollen.  Their 
honey  is  nectar  elaborated  in  the  crop  by  an  unknown 
process ;  while  the  wax  is  secreted  from  the  sides  of  the 
abdomen  and  mixed  with  saliva.  There  is  a  subdivision 


120      STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 

of  extra  labor  :  thus  there  are  wax  workers,  masons,  and 
nurses.  Ants  (except  the  Saiiba)  have  but  two  classes 
of  workers.  While  ants  live  in  hollow  trees  or  subterra- 
nean chambers  (called  formicarium\  honey  bees  and 
wasps  construct  hexagonal  cells.  The  comb  of  the  bee 
is  hung  vertically,  that  of  the  wasp  is  horizontal. 

CLASS  5.  —  Arachnida 

The  arachnids  are  closely  related  to  the  crustaceans, 
having  the  body  divided  into  a  cephalothorax  and  abdo- 
men.22 To  the  former  are  attached  eight  legs  of  seven 
joints  each ;  the  latter  has  no  locomotive  appendages. 
The  head  carries  two,  six,  or  eight  eyes,  smooth  and  ses- 
sile (i.e.,  not  faceted  and  stalked,  as  in  the  lobster),  and 
approaching  the  eye  of  the  vertebrates  in  the  complete- 
ness and  perfection  of  their  apparatus.  There  are  no 
antennae,  the  first  pair  of  appendages  on  the  cephalo- 
thorax being  modified  into  grasping  organs.  They  are 
all  air  breathers,  having  spiracles  which  open  either  into 
air  sacs  or  tracheae.  The  young  of  the  higher  forms  un- 
dergo no  metamorphosis  after  leaving  the  egg. 

Arachnids  number  nearly  five  thousand  species.  The 
typical  forms  may  be  divided  into  three  groups  :  — 

i.  Scorpionida,  or  scorpions,  characterized  by  very 
large  maxillary  palpi  ending  in  forceps,  and  a  prolonged, 
jointed  post-abdomen.  The  nervous  and  circulatory  sys- 
tems are  more  highly  organized  than  those  of  spiders ; 
but  the  long,  tail-like  post-abdomen  and  the  abnormal 
jaws  place  them  in  a  lower  rank.  The  abdomen  consists 
of  twelve  segments :  the  anterior  half  is  as  large  as  the 
thorax,  with  no  well-marked  division  between  ;  the  other 
part  is  comparatively  slender,  and  ends  in  a  hooked  sting, 
which  is  perforated  by  a  tube  leading  to  a  poison  sac. 
The  mandibles  are  transformed  into  small,  nipping  claws, 
and  the  eyes  generally  number  six.  Respiration  is  car- 


ARTHROPODA 


121 


ried  on  by  four  pairs  of  pulmonary  sacs  which  open  on 
the  under  surface  of  the  abdomen.  The  heart  is  a 
strong  artery,  extending  along  the  middle  of  the  back, 
and  divided  into  eight  separate  chambers.  Scorpions 
are  confined  to  the  warm-temperate  and  tropical  regions, 
usually  lurking  in  dark,  damp  places. 


FIG.  80.  —  Scorpion  (under  surface)  and  Centipede. 

2.  Phalangida,    the    harvest   men,   or    "  granddaddy 
longlegs "    (P/ialangium),    frequently    seen    about    our 
houses,  belong  to  this  order.     They  have  a  short,  thick, 
unsegmented  body,  extremely  long  legs,  and  no  spinning 
glands. 

3.  Araneida,  or  spiders.     They  are  distinguished  by 
their  soft,  unjointed  abdomen,   connected  to  the  thorax 
by  a  narrow  neck,  and  provided  at  the  posterior   end 
with  two  or  three  pairs  of  appendages,  called  "spinner- 
ets," which  are  homologous  with  legs.     The  office   of 
the  spinnerets  is  to  reel  out  the  silk  from  the  silk  glands, 


122     STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

the  tip  being  perforated  by  a  myriad  of  little  tubes, 
through  which  the  silk  escapes  in  excessively  fine  threads. 
An  ordinary  thread,  just  visible  to  the  naked  eye,  is  the 
union  of  a  thousand  or  more  of  these  delicate  streams  of 
a  fluid  which,  like  collodion,  hardens  on  exposure  to  the 
air.23 

The  mandibles  are  vertical,  and  end  in  a  powerful 
hook,  in  the  end  of  which  opens  a  duct  from  a  poison 


FIG.  81.  —  A,  female  Spider;  B,  male  of  same  species;  C,  arrangement  of  the  eyes. 

gland  in  the  head  (Fig.  216).  The  maxillae,  or  "  palpi," 
which  in  scorpions  are  changed  to  formidable  claws,  in 
spiders  resemble  the  thoracic  feet,  and  are  often  mis- 
taken for  a  fifth  pair.  The  brain  is  of  larger  size,  and 
the  whole  nervous  system  more  concentrated  than  in  the 
preceding  order.  There  are  generally  eight  simple 
eyes,  rarely  six.  They  breathe  both  by  tracheae  and 


ARTHROPODA  123 

lunglike  sacs,  from  two  to  four  in  number,  situated  under 
the  abdomen.  All  the  species  are  carnivorous. 

The  instincts  of  spiders  are  of  a  high  order.  They 
are,  perhaps,  the  most  wily  of  arthropods.  They  display 
remarkable  skill  and  industry  in  the 
construction  of  their  webs ;  and  some 
species  (called  "mason  spiders")  even 
excavate  a  subterranean  pit,  line  it  with 
their  silken  tapestry,  and  close  the  en- 
trance with  a  lid  which  moves  upon  a 

,    .  n\  ritf.    02.  —  spinnerets 

hinge/*  of  the  spider,  b,c;  a, 

4.  Acarida,  represented  by  the  mites     palpiform  organs' 
and  ticks.    They  have  an  ov-al  or  rounded  body,  without 
any  marked  articulations,  the  head,  thorax,  and  abdomen 
being  apparently  merged  into  one.    They  have  no  brain  ; 

only     a     single     ganglion 
lodged    in    the    abdomen. 
They  breathe  by  tracheae 
FIG.  83. -A  Mite  (DemodexfoincJtio-     ™  through  the  skin.     The 

rum),  one  of  the  lowest  Arachnids;  a         niOUth     is    formed    for    SUC- 
parasite  in  human  hair  sacs ;    X  125. 

tion,  and  they  are  generally 

parasitic.  The  mites  (Sarcoptes)  are  among  the  lowest 
of  articulates.  The  body  is  soft  and  minute.  The  ticks 
(Ixodes)  have  a  leathery  skin,  and  are  sometimes  half 
an  inch  long.  The  mouth  is  furnished  with  a  beak 
for  piercing  the  animal  it  infests. 

5.  Xiphosura,  Arachnida  with  a  broad  carapace  cov- 
ering the  cephalothorax,  an  abdomen  consisting  of  seven 
firmly  united  segments  ending  with  a  long  slender  tail 
of  one  piece,  five  pairs  of  legs  on  the  cephalothorax; 
the   abdomen   with   five   pairs    of    platelike   respiratory 
organs  covered  anteriorly  by  an  operculum.     The  king 
crab    or    horseshoe    crab   (Limtilus),  found    on    muddy 
bottoms  along  the  coast,  belongs  in  this  order,  which  is 
interesting  as  containing  the  only  living  representatives 


124     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

of  the  extinct  trilobites.  Limulus  was  formerly  classified 
among  the  Crustacea,  but  is  now  considered  to  have  its 
closest  affinities  among  the  Arachnida. 

Branch  XI.  —  MOLLUSC  A 

A  mollusk  is  a  soft-bodied  animal,  without  internal 
skeleton,  and  without  segmentation  of  body  or  of  parts, 
covered  with  a  moist,  sensitive,  contractile  skin,  which, 
like  a  mantle,  loosely  envelops  the  creature.  In  some 
cases  the  skin  is  naked,  but  generally  it  is  protected  by 
a  calcareous  covering  (shell).  The  length  of  the  body 
is  less  in  proportion  to  its  bulk  than  in  other  animals. 
The  lowest  class  has  no  distinct  head.  The  nervous 
system  consists  of  three  well-developed  pairs  of  ganglia, 
which  are  principally  concentrated  around  the  entrance 
to  the  alimentary  canal,  forming  a  ring  around  the 
throat.  The  other  ganglia  are,  in  most  cases,  scattered 
irregularly  through  the  body,  and  in  such  the  body  is 
unsymmetrical  (Figs.  331,  332).  The  digestive  system 
is  greatly  developed,  especially  the  "  liver,"  as  in  many 
aquatic  animals  (Figs.  242,  243).  Except  in  the  cepha- 
lopods,  the  muscles  are  attached  to  the  skin,  or  shell. 
There  is  a  heart  of  two  chambers  (auricle  and  ventricle) 
or  three  (two  auricles  and  ventricle).  As  in  all  inverte- 
brates, the  heart  is  arterial.  In  mollusks,  with  rare 
exceptions,  we  find  no  repetition  of  parts  along  the 
antero-posterior  axis.  They  are  best  regarded  as 
"worms"  of  few  segments,  which  are  fused  together 
and  much  developed.  The  total  number  of  living  species 
probably  exceeds  twenty  thousand.  The  great  majority 
are  water  breathers,  and  marine ;  some  are  fluviatile  or 
lacustrine,  and  a  few  are  terrestrial  air  breathers.  All 
bivalves,  and  nearly  all  univalves,  are  aquatic.  Each 
zone  of  depth  in  the  sea  has  its  particular  species.  The 
most  important  classes  are  now  to  be  described. 


MOLLUSCA 


125 


CLASS  i.  —  Pelecypoda 

These  mollusks,  formerly  called  lamellibranchs,  are 
all  ordinary  bivalves,  as  the  oyster  and  clam.  The 
shells  differ  from  those  of  brachiopods  in  being  placed 
on  the  right  and  left  sides  of  the  body, 
so  that  the  hinge  is  on  the  back  of  the 
animal,  and  in  being  unequilateral  and 
equivalved.25  The  umbo,  or  beak,  is 
the  point  from  which  the  growth  of  the 
valve  commences.  Both  brachiopods 
and  pelecypods  are  headless;  but  in  FIG.  84. -Pearl  Oyster 

.          ,  .  (Meleagrina  mar  gar  i- 

tne  latter  the  mouth  points  the  same  u/eray,  one  fourth  nat- 
way  as  the  umbo,  i.e.,  toward  the  uralsize"  Ceylon' 
anterior  part.  The  length  of  the  shell  is  measured 
from  its  anterior  to  its  posterior  margin,  and  its  breadth 
from  the  dorsal  side,  where  the  hinge  is,  to  the  opposite, 
or  ventral,  edge.  The  valves  are  united  to  the  animal 
by  one  muscle  (as  in  the  oyster),  or  two  (as  in  the  clam), 
and  to  each  other  by  a  hinge.  In  some  species,  as  some 
fresh-water  mussels,  the  hinge  is  simply  an  elastic  liga- 
ment, passing  on  the  outside  from  one 
valve  to  the  other  just  behind  the  beak, 
so  that  it  is  stretched  when  the  valves 
are  closed.  Another  is  placed  between 
the  edges  of  the  valves,  so  that  it  is 
squeezed  as  they  shut,  like  the  spring 
in  a  watch  case.  Such  bivalves  are 
said  to  be  edentulous.  But  in  the 
majority,  as  the  clam  and  the  fresh- 
water Unio,  the  valves  also  articulate 
by  interlock-parts  called  teeth.  The 
valves  are,  therefore,  opened  by  the 
ligaments,  and  closed  by  the  muscles. 
The  shell  is  secreted  by  the  mantle. 


FIG.  85.  —  Salt  -  water 
Mussel  (Mytilus  pel- 
lucidus).  Atlantic 
coasts. 


126     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

Lamellibranchs  breathe  by  four  hollow,  platelike  gills 
(whence  the  name),  two  on  each  side  underneath  the 
mantle  (Fig.  275),  the  water  being  drawn  into  the  cavi- 
ties in  the  gills  by  the  action  of  ciliated  cells.  In  the 
higher  forms,  the  margin  of  the  mantle  is  rolled  up  into 
two  tubes,  or  siphons,  for  the  inhalation  and  exhalation 
of  water.  They  feed  on  microscopic  organisms  gathered 
from  the  water  by  the  ciliated  inner  surface  of  the  mantle, 
the  cilia  producing  a  flow  of  particles  toward  the  mouth. 


CL 

FIG.  86.  —  Lamellibranch  (Mactra):  a,  foot;  b,  c,  siphons. 

A  few  are  fixed ;  the  oyster,  e.g.  habitually  lying  on  its 
left  valve,  and  the  salt-water  mussel  hanging  to  the  rocks 
by  a  cord'  of  threads  called  "  byssus  "  ;  but  most  have  a 
"  foot,"  by  which  they  creep  about.  Unlike  the  oyster, 
also,  the  majority  live  in  an  erect  position,  resting  on  the 
edges  of  their  shells.  About  five  thousand  living  species 
are  known.  These  are  fresh-water  and  marine,  and  range 
from  the  shore  to  a  depth  of  a  thousand  feet. 

The  chief  characters  for  distinguishing  lamellibranchs 
are  the  muscular  impressions,26  whether  one  or  two ;  the 
presence  of  a  pallial  sinus,  which  indicates  the  possession 
of  siphons ;  the  structure  of  the  gills,  and  the  symmetry 
of  the  valves  (Fig.  296). 

The  following  are  the  more  important  orders,  classi- 
fied according  to  gill  structure  :  — - 


MOLLUSCA  127 

1.  Filibranchia,  with  two  pairs  of  platelike  gills,  the 
filaments  being  V-shaped,  usually  two  adductor  muscles 
of  which  the  anterior  is  often  the  smaller  or  may  even 
be  absent,  sea  mussel  (Mytilns)  (Fig.  85). 

2.  Pseudo-lame  llibranchia,  with  gills  showing  vertical 
folds,  a  single,  large  (posterior)  adductor  muscle,  the 
shell  frequently  inequivalve,  oyster  (Ostrea)  (Fig.  242), 
scallop  (Pec fen),  pearl  oyster  (Meleagrina)  (Fig.  84). 

3.  Eulamellibranchia,  with  gills  smooth  or  vertically 
plaited  and  with  two  adductor  muscles  of  equal  size, 
fresh-water  mussel  (Unio  and  Anodontd),  cockle  (Car- 
dium)  (Fig.  87),  quahog  (Venus),  shipworm  (Teredo), 
and  common  clam   (Mya).27 

CLASS  2. — :Amphineura 

The  animals  in  this  class  were  formerly  placed  among 
the  Gastropoda,  but  are  now  considered  to  be  sufficiently 
distinct  to  be  grouped  by  them- 
selves. They  are  bilaterally 
symmetrical,  elongated  mollusks, 
with  a  shell  consisting  of  eight 
separate  pieces,  or  else  entirely 
lacking.  The  mantle  is  not  di- 
vided into  paired  lobes  as  in  the 
bivalves.  Chiton,  a  sluggish  ani- 
mal with  the  habit  of  the  limpet, 

FIG.  87. — Cockle  (Cardznm  cos- 
1S    One     Of     the     beSt-knOWn    forms        tatuni)  \  one  third  natural  size. 

(Fig.  ioo).     The  shell-less  mem-     Chinaseas- 

bers  of  the  class  are  the  lowest  in  organization  of  all 

of  the  mollusks. 

CLASS  3.  —  Gastropoda 

The  snails  are,  with  rare  exceptions,  all  univalves.28 
The  body  is  coiled  up  in  a  conical  shell,  which  is  usually 
spiral,  the  whorls  passing  obliquely  (and  generally  from 


128     STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

right  to  left),29  around  a  central  axis,  or  "  columella " 
(Fig.  297).  When  the  columella  is  hollow  (perforated), 
the  opening  in  the  end  is  called  the  "  umbilicus."  When 
the  whorls  are  coiled  around  the  axis  in  the  same  plane, 
we  have  a  discoidal  shell,  as  the  Planorbis.  The  mouth, 
or  "  aperture,"  of  the  shell  is  "  entire  "  in  most  vegetable- 
feeding  snails,  and  notched  or  produced  into  a  canal  for 


FIG.  88.  — Whelk  {Buccinutri) ,  showing  operculum,  o,  and  siphon,  s. 

the  siphons  in  the  carnivorous  species.  The  former  are 
generally  land  and  fresh-water  forms,  and  the  latter  all 
marine.  In  some  gastropods,  as  the  river  snails  and 
most  sea  snails,  a  horny  or  calcareous  plate  (operculum} 
is  secreted  on  the  foot,  which  closes  the  aperture  when 
the  animal  withdraws  into  its  shell.  In  locomotion,  the 
shell  is  carried  with  the  apex  directed  backward. 

The  body  of  most  gastropods  is  unsymmetrical,  the 
organs  not  being  in  pairs,  but  single,  and  on  one  side, 
instead  of  central.  The  mantle  is  continuous  around 
the  body,  not  bilobed,  as  in  lamellibranchs.  A  few,  as 
the  common  garden  snail,  have  a  lung;  but  the  vast 
majority  breathe  by  gills.  The  head  is  more  or  less 
distinct,  and  provided  with  two  tentacles,  with  auditory 


MOLLUSCA  I2Q 

sacs  at  their  bases ;  two  eyes,  which  are  often  on  stalks ; 
and  a  .  straplike  tongue  (odontophore),  covered  with 
minute  teeth  (Fig.  227).  The  heart  is  situated,  in  the 
majority,  on  the  right  side  of  the  back,  and  consists  of 
an  auricle  and  a  ventricle  (Fig.  243).  The  nervous 
ganglia  are  united  into  an  esophageal  ring  or  collar 
(Fig.  351).  All,  except  the  pteropods,  move  by  means 
of  a  ventral  disk  or  foot. 

Gastropods  are  now  the  reigning  mollusks,  comprising 
three  fourths  of  all  the  living  species,  and  are  the  types 
of  the  branch.  They  have  an  extraordinary  range  in 
latitude,  altitude,  and  depth. 

Omitting  a  few  rare  and  aberrant  forms,  we  may  sepa- 
rate the  class  into  the  following  orders :  — 

1.  Aspidobranchia,    gastropods    having   a    somewhat 
diffuse  nervous  system,  the  cerebral  ganglia  being  wide 
apart,  two  auricles  in  the  heart,  gills  plumelike,  limpet 
(Patella,  Fig.  105),  well  known  to  every  seaside  visitor, 
and  the  beautiful  ear-shell  (Haliotis,  Fig.  95),  frequently 
used  for   ornaments   and  inlaid   work,    the    pyramidal 
Trochus,  and  the  pearly  Turbo  (Fig.  102). 

2.  Pectinibranchia,  gastropods  with  a  somewhat  con- 
centrated nervous  system,  heart  with  a  single  auricle, 
gill  bearing  a  single  row  of  lamellae  and  attached  to  the 
wall  of  the  mantle.     This  order  includes  many  of  the 
most  beautiful  of  the  sea  shells,  the  cowry  (Cyprcza) 
(Fig.  94),  cones  (Fig.  99),  whelk  (Buccinum)  (Fig.   88), 
trumpet  shell  (Triton),  volute  (Fig.    101),  olive,  harp, 
cameo  shell  (Cassis)  (Fig.  97),  rock  skell(Mupex),  spindle 
shell  (Fus2is)  (Fig.  96),  and  wing  shell  (Strombus)(¥ig. 
103).     All  of    these   are  marine.     Many   of  them  are 
carnivorous  and  have  the  margin  of  the  shell  notched. 

3.  Opisthobranchia.    The  pteropods  are  small,  marine, 
floating  mollusks,  whose  main  organs  of  motion  resem- 
ble a  pair  of   wings  or  fins,   coming  out  of  the  neck, 

DODGE'S  GEN.  ZOOL.  —  9 


130     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


whence  the  common  name,  "  sea  butterflies."  Many 
have  a  delicate,  transparent  shell.  The  head  has  six 
appendages,  armed  with  several 
hundred  thousand  microscopic  suck- 
ers —  a  prehensile  apparatus  un- 
equalled in  complication.  Ptero- 
pods  occur  in  every  latitude,  but 
generally  in  mid-ocean,  and  in  the 
arctic  regions  are  the  food  of  whales 

FIG.  89.  -  A  Pteropod  (Hya-      an(J   gea  birds. 
laa  tridentata) .     Atlantic. 

The    sea   hare   (Aplysia),  which 

discharges  a  purple  fluid,  and  the"  bubble  shell  (Build) 
belong  here. 

The  nudibranchs  or 
sea  slugs  are,  for  the 
most  part,  naked  mol- 
lusks,  only  a  few  hav- 
ing a  Shell.  They  are  FIG.  9o.-A  Tritonian  (Dendronotus  arbores- 

found  in  all  seas,  from 

the  arctic  to  the  torrid,  generally  on  rocky  coasts. 
When  disturbed,  most  of  them  draw  themselves  up  into 
a  lump  of  jelly  or  tough  skin.  Ex- 
amples :  sea  lemon  (Doris),  the  beau- 
tiful Tritonia,  and  the  painted  sEolis. 
4.  Pulmonata.  —  These  air  breath- 
ing gastropods,  represented  by  the 
familiar  snail,  have  the  simplest 
form  of  lung  —  a  cavity  lined  with 
a  delicate  network  of  blood  vessels, 
which  opens  externally  on  the  right  side  of  the  neck. 
This  is  the  mantle  cavity.  The  entrance  may  be  closed 
to  shut  out  the  water  in  the  aquatic  tribes,  and  the  hot, 
dry  air  of  summer  days  in  the  land  species.  They  are 
all  fond  of  moisture,  and  are  more  or  less  slimy.  Their 
shells  are  lighter  (being  thinner,  and  containing  less 


FIG.  91.  —  Bulla  ampulla, 
or  "bubble  shell";  three 
fourths  natural  size.  In- 
dian Ocean. 


MOLLUSCA 


earthy  matter)  than  those  of  marine  mollusks,  having 
to  be  carried  on  the  back  without  the  support  of  the 


FIG.  92.—  A,  Land  Suail  (Helix)',  B,  C,  D,  Slugs  (Limax};  E,  F,  G,  Pond  Snails 
(Lzmnaa,  Paludina,  and  Planorbis). 

water.     Their  eggs  are  laid  singly,  while  the  eggs  of 
other  orders  are  laid  in  chains. 

They  are  found  in  all  zones,  but 
are  most  numerous  where  lime  and 
moisture  abound.  All  feed  on  vege- 
table matter.  A  few  are  naked,  as 
the  slug  ;  some  are  terrestrial  ;  others 
live  in  fresh  water.  The  land  snails, 
represented  by  the  common  Helix,  the 
gigantic  Bidimus  (Strcphocheilus),  and 
the  slug  (Limax),  are  distinguished  by 
their  four  "horns,"  the  short  front  pair  FIG.  93.  —  £«&>« 
being  the  true  tentacles,  and  the  long 
hinder  pair  being  the  eye  stalks.  They 
have  a  sawlike  upper  jaw  for  biting  leaves,  and  a  short 
tongue  covered  with  minute  teeth.  The  pond  snails. 


Guiana- 


132     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


FIG.  94.  —  Cowry  (Cyprcea  capensis) ;  two 
thirds  natural  size.    South  Africa. 


FIG.  95.  —Haliotis,  or  "  Pearly  Ear  Shell. 
Pacific  coasts. 


FIG.  96.  —  Spindle  Shell 
(Fusus  colus);  one 
half  natural  size. 
Ceylon. 


FIG.  97.  —  Cassis  rufa,  or 
"  Helmet  Shell;  "  one  fourth 
natural  size.  Indian  Ocean. 


FIG.  98.  —  Auger  Shell 
( Terebra  maculata) ; 
one  half  natural  size. 
China  seas. 


FlG.  99.  —  Cone  Shell  (Conns 
marmoreus} ;  two  thirds 
natural  size.  China  seas. 


FIG.  100.  —  Chiton  squa- 
mosus;  one  half  natural 
size.  West  Indies. 


FIG.  101. — Volute  (Valuta 
mnsica) ;  one  half  natu- 
ral size.  West  Indies. 


MOLLUSCA 


133 


FIG.  102.  — Top  Shell  (Turbo  marnto- 
ratus) ;  one  fourth  natural  size. 
Australia. 


FIG.  103. — Strombus  gigas,   or   ''Wing 
Shell";    one  fifth   natural   size.      West 


FIG.  104.  —  Paludina,  a  fresh-water 
snail. 


FIG.  105.  —  Key-hole  Limpet  (Fissurella 
listen"} .     West  Indies. 


FIG.  106.  —  Ear  Shell  (//.  tuberculata} ,  and  Dog  Whelk  (Nassa  reticulata) .    England. 


134     STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

as  Limncea  and  Planorbis,  differ  in  having  no  eye  stalks, 
the  eyes  being  at  the  base  of  the  tentacles.  They  are 
obliged  to  come  frequently  to  the  surface  of  the  water 
to  breathe. 

CLASS  4.  — -  Cephalopoda 

The  cephalopods  stand  at  the  head  of  the  branch. 
The  head  is  set  off  from  the  body  by  a  slight  constric- 
tion, and  furnished  with  a  pair  of  large,  staring  eyes,  a 
mouth  armed  with  a  rasping  tongue  and  a  parrotlike 
beak,  and  eight  or  more  tentacles  or  arms.  The  body 
is  symmetrical,  and  wrapped  in  a  muscular  mantle.  The 
shell,  if  present,  may  be  internal  or  external  (Fig.  245). 
The  nervous  system  is  more  concentrated  than  in  other 

invertebrates ;  the  cerebral 
ganglia  are  partly  inclosed 
in  a  cartilaginous  cranium. 
All  the  five  senses  are 
present.  The  class  is  en- 
tirely marine  (breathing  by 
plumelike  gills  on  the  sides 
of  the  body),  and  carnivo- 
rous. The  naked  species 
are  found  in  every  sea. 
Those  with  chambered  shells 
(as  Nautilus,  Ammonites,  and 
Orthoceras)  were  once  very 
abundant;  more  than  two 
thousand  fossil  species  are 
known, butonly  three  species 
have  been  found  living. 

I .  Dibranchs.  —  These  are 
the  most  active  of  mollusks, 
FIG.  107.— Cuttlefish  (Sepia  officinaiis}-,  and  the  tyrants  of  the  lower 

one  fifth  natural  size.     Atlantic  coasts.        ,     .-i  «  ,-1  .1 

tribes.    Among  them  are  the 
largest  of  invertebrate  animals.     They  are  naked,  having 


MOLLUSCA 


135 


no  external  shell  covering  the  body,  but  usually  a  horny  or 
calcareous  part  within.  They  have  a  distinct  head,  promi- 
nent eyes,  horny  mandibles,  eight  or  ten  arms  furnished 
with  suckers,  two  gills,  a  complete  tubular  funnel,  and  an 
ink  bag  containing  a  peculiar  fluid  (sepia},  of  intense 
blackness,  with  which  the  water  is  darkened  to  facilitate 
escape.  They  have  the  power  of 
changing  color,  like  the  chame- 
leon. They  crawl  with  their  arms 
on  the  bottom  of  the  sea,  head 
downward,  and  also  swim  back- 
ward or  forward,  usually  with  the 
back  downward,  by  means  of  fins, 
or  squirt  themselves  backward  by 
forcing  water  forward  through 
their  breathing  funnels.  ;  - 

The  paper  nautilus  (Argonautd) 
and  the  poulpe  (Octopus)  have  eight 
arms.  The  female  argonaut  se- 
cretes a  thin,  unchambered  shell 
for  carrying  its  eggs.  The  squid 
(Loligo)  and  cuttlefish  (Sepia)  have  FIG.  108. 

1  -.  -. .    .  ,  , 

ten  arms,  the  additional  pair  be- 
ing much  longer  than  the  others. 
Their  eyes  are  movable,  while  those 
of  the  argonaut  and  poulpe  are 
fixed.  The  squid,  so  much  used 
for  bait  for  cod,  has  an  internal 
horny  "pen,"  and  the  cuttle  has  a  spongy,  calcareous 
"  bone."  The  extinct  Belemnites  had  a  similar  structure. 
Squid  have  been  found  with  a  body  eleven  feet  and 
arms  thirty-nine  feet  long,  and  parts  of  others  still 
larger  —  as  much  as  seventy  feet  in  total  length. 

2.    Tetrabranchs.  —  This  group  is  characterized  by  the 
possession  of  four  gills,  forty  or  more  short  tentacles, 


yeatti)  with 
the  mantle  cut  open  :  b,  bran- 
chial heart ;  e,  eye ;  f,  fin ;  g, 
gill;  /',  intestine;  ib,  ink  bag; 
m,  cut  edge  of  mantle;  ma, 
mantle  artery;  me,  mantle 
cavity;  met,  mantle  carti- 
lages; pvc,  posterior  vena 
cava;  s,  siphon;  t,  tentacles 
with  sucking  disks;  us,  visce- 
ral sac. 


136     STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 

and  an  external,   chambered  shell.     The  partitions,  or 
septa,  of  the  shell  are  united  by  a  tube  called  "  siphun- 


FIG.  109.  —  Female  paper  nautilus  (A  rgonauta  argo) :  i,  swimming  toward  a  by  ejecting 
water  from  funnel,  b  ;  2,  crawling  on  the  bottom;  3,  coiled  within  its  shell,  which  is 
one  fourth  natural  size.  Mediterranean. 

cle,"    and   the    animal    lives    in    the   last    and   largest 
chamber.33     The   living  nautilus  has  a  smooth,  pearly 


FIG.  no.  —  Pearly  nautilus,  with  shell  bisected;  one  half  natural  size.     Indian  Ocean. 

shell,  a  head    retractile  within  the  mantle   or  "hood," 
and  calcareous  mandibles,  well  fitted   for   masticating 


CHORDATA 


137 


crabs,  on  which  it  feeds.  The  pearly  nautilus  dwells  in 
the  Indian  Ocean,  crawling  on  the  bottom  at  moderate 
depths;  and,  while  the  shell  is  well  known,  only  a  few 
specimens,  comparatively,  of  the  animal  have  ever  been 
obtained. 

Branch  XII.  —  CHORDATA 

This  grand  division  includes  the  most  perfect  animals, 
or  such  as  have  the  most  varied  functions  and  the  most 
perfect  and  complex  organs.     Besides  the  unnumbered 
host  of  extinct  forms, 
there  are  about  twen- 
ty-five  thousand    liv- 
ing    species,    widely 
differing  among  them- 
selves in   shape   and 
habits,  yet  closely  al- 
lied    in     the      grand 
features  of   their  or- 
ganization,   the   gen- 
eral type  being  end- 
lessly modified. 

The  fundamental 
distinctive  character 
of  typical  chordates 

is    the     Separation     Of  FIG.  in.  -  Ideal  plans  of  the  branches.      V.  trans- 

,                 .                           ,.       ,  verse   section    of  vertebrate   type;     v,  the  same 

tne   mam   maSS  OI    tne  inverted.      M,    transverse  section  of  molluscous 

nprvniic;    <;v<;t^m    from  type;  and  A/i/,  of  molluscoid.     A  and  Ad,  trans- 

sysieni  yerse  sections  of  articulate  type>  high  and  low. 

the    general    Cavity    Of  C>  longitudinal    section  of  ccelenterate   type;  a, 

alimentary  canal ;   c,  body  cavity.     In  the  other 

the     body.        A     tranS-  figures,  the  alimentary  canal  is  shaded,  the  heart 

, .  £    ,  i_  is  black,  and  the  nervous  cords  are  open  rings. 

verse   section   of  the 

body  exhibits  two  cavities,  or  tubes  —  the  dorsal,  con- 
taining the  cerebrospinal  nervous  system ;  the  ventral, 
inclosing  the  alimentary  canal,  heart,  lungs,  and  a 
double  chain  of  ganglia,  or  sympathetic  system.  This 


STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 

ventral,  or  hemal,  cavity  cor- 
responds to  the  whole  body 
of  an  invertebrate;  while 
the  dorsal,  or  neural,  is 
mainly  additional. 

Vertebrates  are  also  dis- 
tinguished by  an  internal, 
jointed  skeleton,  endowed 
with  vitality,  and  capable  of 
growth  and  repair.  During 
embryo  life  it  is  represented 
by  the  notochord  ;  but  in  the 
higher  forms  this  is  after- 
ward replaced  by  a  more 
highly  developed  vertebral 
column  of  cartilage  or  bone. 
The  column  and  cranium 
are  never  absent  in  the  Cra- 
niata;  other  parts  may  be 
wanting,  as  the  ribs  in  frogs, 
limbs  in  snakes,  etc.  The 
limbs  are  never  more  than 
four,  and  are  always  articu- 
lated to  the  hemal  side  of  the 
body,  while  the  legs  of  inver- 
tebrates are  developed  from 
the  neural  side.  The  mus- 
cles moving  the  limbs  are  at- 
tached to  the  endo-skeleton. 

The  circulation  of  the 
blood  is  complete,  the  arte- 
ries being  joined  to  the  veins 

FIG.  112. — Diagram  of  circulation  in  the      ,  .,,  ,  , 

higher  vertebrates:   i,  heart;   2,  lungs;       by     CaplllariCS,      SO     that     the 

J\»TS^Ti±iS  blood    never  escaPes    into 

tremities;    8,   liver.      (From  Dalton's      the  visceral   Cavity   as   in   the 
"  Physiology.") 


CHORDATA  139 

invertebrates.  All  have  a  portal  vein,  carrying  blood 
through  the  liver;  all  have  lacteals  and  lymphatics. 
The  blood  is  red,  and  contains  both  kinds  of  corpuscles. 
The  teeth  are  developed  from  the  dermis,  never  from 
the  cuticle,  as  in  mollusks  and  arthropods;  the  jaws 
move  vertically,  and  are  never  modified  limbs.  Except 
in  the  lowest  forms  the  liver  and  kidneys  are  always 
present.  The  respiratory  organs  are  either  gills  or 
lungs,  or  both.  Vertebrates  are  the  only  animals  which 
breathe  through  the  mouth  cavity. 

The  nervous  system  has  two  marked  divisions  :  the 
cerebrospinal,  presiding  over  the  functions  of  animal 
life  (sensation  and  locomotion);  and  the  sympathetic, 
which  partially  controls  the  organic  functions  (digestion, 
respiration,  and  circulation).  .  In  no  case  does  the  gullet 
pass  through  the  nervous  system,  as  in  invertebrates, 
and  the  mouth  opens  on  the  side  opposite  to  the  brain. 
Except  in  the  lowest  members  of  this  group  probably 
none  of  the  five  senses  is  ever  altogether  absent.  The 
form  of  the  brain  is  modified  by  the  relative  develop- 
ment of  the  various  lobes.  In  the  lower  vertebrates, 
the  cerebral  hemispheres  are  small  —  in  certain  fishes 
they  are  actually  smaller  than  the  optic  lobes  —  in  the 
higher,  they  nearly  or  quite  overlap  both  olfactories 
and  cerebellum.  The  brain  may  be  smooth,  as  in  most 
of  the  cold-blooded  animals,  or  richly  convoluted,  as  in 
man. 

There  is  no  skull  in  Amphioxus.  In  the  Cyclosto- 
mata  and  Elasmobranchii  it  is  cartilaginous.  In  other 
fishes  it  is  cartilage  overlaid  with  bone.  In  amphibians 
and  reptiles,  it  is  mingled  bone  and  cartilage.  In  birds 
and  mammals,  it  is  mainly  or  wholly  bony.  The  human 
skull  contains  fewer  bones  than  the  skull  of  most 
animals,  excepting  birds.  The  skull  of  all  vertebrates 
is  divisible  into  two  regions  :  the  cranium,  or  brain  case, 


140     STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 

and  the  face.  The  size  of  the  cranial  capacity,  com- 
pared with  the  area  of  the  face,  is  generally  the  ratio 
of  intelligence.  In  the  lower  orders,  the  facial  "part  is 
enormously  predominant,  the  eye  orbits  are  directed 
outward,  and  the  occipital  condyles  are  nearly  on  a  line 
with  the  axis  of  the  body.  In  the  higher  orders,  the 
face  becomes  subordinate  to  the  cranium,  the  sensual 
to  the  mental,  the  eyes  look  forward,  and  the  condyles 
approach  the  base  of  the  cranium.  Compare  the 
"snouty"  skull  of  the  crocodile,  and  the  almost  vertical 
profile  of  civilized  man.  A  straight  line  drawn  from 
the  middle  of  the  ear  to  the  base  of  the  nose,  and 
another  from  the  forehead  to  the  most  prominent  part 
of  the  upper  jaw,  will  include  what  is  called  \he,  facial 
angle,  which  roughly  gives  the  relation  between  the  two 
regions,  and  the  intellectual  rank  of  the  animal.31  In 
the  cold-blooded  vertebrates  the  brain  does  not  fill  the 
cranium  ;  while  in  birds  and  mammals  a  cast  of  the 
cranial  cavity  well  exhibits  the  general  features  of 
the  cerebral  surface.32 

All  higher  vertebrates  are  single  and  free.  Mammals 
bring  forth  their  young  alive,  the  young  before  birth 
deriving  their  nourishment  directly  from  the  mother 
(viviparous).  In  almost  all  the  others  the  nourishment 
is  stored  up  in  the  egg,  which  is  laid  before  hatching 
(oviparous),  or  is  retained  in  the  mother  until  hatched 
(ovoviviparons\  as  in  some  reptiles  and  fishes. 

Of  the  branch  Chordata  there  are'  three  subbranches  : 
Adelochorda,  Urochorda,  and  Vertebrata.  The  first  in- 
cludes Balanoglossus,  a  wormlike  creature  regarded  by 
some  zoologists  as  being  related  to  the  backboned 
animals,  together  with  two  other  forms  (Rhabdopleura 
and  Cephalodiscus)  whose  affinities  are  less  plain.  The 
second  includes  the  tunicates,  while  the  great  mass  of  the 
Chordata  belong  in  the  third  subdivision  of  the  branch. 


ADELOCHORDA 


141 


The  group  Vertebrata  consists  of  two  divisions,  the  first, 
Acrania,  including  the  skull-less  forms,  e.g.,  the  lancelet 
(Amphioxus),  while  the  second,  and  much  larger  divi- 
sion, Craniata,  consists  of  six  great  classes,  Cyclostomata, 
Pisces,  Amphibia,  Reptilia,  Aves,  and  Mammalia.  The 
first  four  are  "cold-blooded,"  the  other  two  are  "  warm- 
blooded." Cyclostomes,  fishes,  and  amphibians  have 
gills  during  the  whole  or  a  part  of  their  lives,  while  the 
rest  never  have  gills.  Fishes  and  amphibians  in  embryo 
have  neither  amnion  nor  allantois,  while  the  animals  in 
the  last  three  classes  are  provided  with  both. 

The  skull  bearing  vertebrates  may  be  grouped  into 
three  provinces. 

Cyclostomes,  fishes,  and  amphibians  agree  in  having 
gills  or  gill  pouches,  in  wanting  amnion  and  allantois, 
and  in  possessing  nucleated  red  blood  corpuscles 
(Ichthyopsida ). 

Birds  and  reptiles  agree  in  having 
no  gills,  but  both  amnion  and  allan- 
tois, in  the  articulation  of  the  skull 
with  the  spine  by  a  single  condyle, 
in  the  development  from  the  skin  of 
feathers  or  scales,  and  in  having 
oval,  nucleated,  red  corpuscles  (Sau- 
ropsi'dd). 

Mammals  differ  from  birds  and 
reptiles  in  having  two  occipital  con- 
dyles,  and  their  red  blood  corpuscles 
are  not  nucleated  33  (Mammalia). 


SUBBRANCH  AND  CLASS  i .  —  Adelochorda 

FIG.  113.  —  Balanoglossus, 

The  principal  representative  of  this     A  proboscis;  c,"  collar"; 

i  •        D    /  /  r.-UJ-j          **,  gill  slits.     Enlarged. 

class  is  Balanoglossus,  a  soft-bodied, 

wormlike  animal,  one  inch  to  six  inches  long,  which  lives 

in  the  sand  and  mud,  along  the  Mediterranean  coast, 


142     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


and  is  also  found  in  the  English  Channel  and  Chesa- 
peake Bay.  It  is  placed  among  the  Chordata  because 
it  is  regarded  by  some  zoologists  as  having  a  notochord, 
gill  slits,  and  a  dorsal  nerve  cord.  All  of  these  are  so 
rudimentary  that  the  position  of  the  animal  in  the 
scheme  of  classification  is  not  yet  definitely  determined. 
Other  structural  and  developmental  features  ally  Bala- 
noglossus  to  the  annelids,  and  the  echinoderms,  for 
which  reason  the  animal  may  be  looked  upon  as  an  inter- 
mediate form  between  these  groups  and  the  Chordata. 


SUBBRANCH  AND  CLASS  2.  —  Urochorda 

The  tunicates  form  a  small  and  singular  group  of 
animals  now  regarded  as  being  the  degenerate  descen- 
dants of  primitive  Chordata. 


FIG.  114.  —  An  ascidian. 


FIG.  115.  —  Diagram  of  a  tunicate,  i, 
inhalent  opening;  bs,  branchial  sac; 
t,  "  tunic";  p,  peribranchial  cavity; 
ce,  esophagus;  s.  stomach;  a,  anus; 
c,  cloaca;  h,  heart;  r,  reproductive 
organs;  «,  nerve  ganglion. 


They  occur  both  as  fixed 
and  as  free  swimming  forms, 
and  as  single  individuals  as 
well  as  chains  or  groups  of  individuals.  The  most 
common  forms  (the  solitary  Ascidians)  are  inclosed  in  a 
leathery,  elastic  bag,  one  end  of  which  is  fastened  to  the 
rocks,  while  the  other  has  two  orifices,  for  the  inlet  and 
exit  of  a  current  of  water  for  nutrition  and  respiration. 


UROCHORDA 


143 


They  are  without  head,  feet,  arms,  or  shell.  Indeed, 
few  animals  seem  more  helpless  and  apathetic  than  these 
apparently  shapeless  beings.  The  tubular  heart  exhibits 
the  curious  phenomenon  of  reversing  its  action  at  brief 
intervals,  so  that  the  blood  oscillates  backward  and  for- 
ward in  the  same  vessels. 
Another  peculiarity  is  the 
presence  of  cellulose  in  the 
skin.  The  water  is  drawn 
by  cilia  into  a  branchial  sac, 
an  enlargement  of  the  first 
part  of  the  intestine,  whence 
it  escapes  through  openings 
in  the  sides,  to  the  excurrent 
orifice,  while  the  particles  of 
food  drawn  in  with  the  water 
are  retained  and  passed  into 
the  intestine.  The  larva  is 
active  for  a  few  hours,  swim- 
ming by  means  of  a  long  tail. 
It  looks  like  a  minute  tad- 
pole, and  has  a  notochord  and 
a  nervous  system  closely  resembling  those  of  a  verte- 
brate. Afterward  it  attaches  itself  by  the  head,  the  tail 
is  absorbed,  and  the  nervous  system  is  reduced  to  a 
single  small  ganglion.  Thus  the  animal,  whose  larval 
structure  is  that  of  a  vertebrate  (since  it  possesses  a 
dorsal  nerve  cord,  a  notochord  in  the  dorsal  region, 
and  gill  slits  opening  to  the  exterior),  degenerates  in 
its  adult  stage  into  an  invertebrate. 

Besides  developing  from  fertilized  eggs,  the  tunicates 
also  multiply  by  the  process  of  budding.  In  Salpa 
and  some  other  kinds,  alternation  of  generations 
takes  place.  All  species  are  marine  and  some  form 
colonies. 


FIG.  116.  —  Larval  stage  of  a  tunicate, 
showing  the  notochord,  « ;  the  spinal 
cord,  c ',  and  the  sucking  disks,  d,  by 
which  the  larva  becomes  attached 
previous  to  changing  to  the  adult 
condition.  Much  magnified. 


144      STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 


SUBBRANCH  3. — Vertebrata 

DIVISION  A.  —  Acrania 
Vertebrates  without  a  skull. 

CLASS.  —  Pharyngobranchii 

The  Acrania  are  represented  by  the  singular  animal 
Amphioxus  or  lancelet.  It  is  about  two  inches  long, 
semitransparent,  without  skull,  limbs,  brain,  heart,  or  red 
corpuscles.  It  has  for  a  skeleton  a  notochord  only.  It 
breathes  by  very  numerous  gill  arches,  without  fringes, 


FIG.  117. — Lancelet  (Amphioxus).  Notochord,  c  ;  spinal  cord,  sc  ;  oral  tentacles,/; 
gills,  g  ;  ovary,  ov  ;  liver,  /  ;  anus,  a  ;  pore  of  branchial  chamber,  p  ;  muscle  plates, 
m  ;  tail  fin,/.  Natural  size. 

and  the  water  is  drawn  in  by  cilia,  which  line  the  gill 
slits.  The  embryo  develops  into  a  gastrula  closely 
resembling  that  of  the  invertebrates.  The  animal  lives 
in  the  sandy  bottom  of  shallow  parts  of  the  ocean,  and 
has  been  found  in  the  Mediterranean  Sea,  in  the  Indian 
Ocean,  and  on  the  coasts  of  North  America  and  South 
America. 

DIVISION  B.  —  Craniata 

Vertebrates  with  a  distinct  skull. 

* 

CLASS  I.  —  Cyclostomata 

The  lampreys 
and  hagfish  have 
a  persistent  noto- 
chord, a  cartilagi- 
nous skull,  no  lower 
jaw,  a  round,  suc- 

Fic.  118.  -  Lamprey  (Petromyzon  marinus) .   Atlantic.      tOlial  niOUth,  homy 


VERTEBRATA  145 

teeth,  one  nasal  organ,  no  scales,  limbs,  or  gill  arches. 
The  gills  are  in  pouches  which  open  separately.  They 
are  found  both  in  salt  water  and  in  fresh  water. 


CLASS  II.  —  Pisces 

Fishes  fall  far  behind  the  rest  of  the  typical  craniates 
in  strength,  intelligence,  and  sensibility.  The  eyes, 
though  large,  are  almost  immovable,  bathed  by  no  tears, 
and  protected  by  no  lids.  Dwelling  in  the  realm  of 
silence,  ears  are  little  needed,  and  such  as  they  have  are 
without  external  parts,  the  sound  being  obliged  to  pass 
through  the  cranium.  Taste  and  smell  are  blunted,  and 
touch  is  nearly  confined  to  the  lips. 

The  class  yields  to  no  other  in  the  number  and  variety 
of  its  forms.  It  includes  nearly  one  half  of  all  the  ver- 
tebrated  species.  So  great  is  the  range  of  variation,  it 
is  difficult  to  frame  a  definition  which  will  characterize 
all  the  finny  tribes.  It  may  be  said,  however,  that  fishes 
are  the  only  backboned  animals  having  median  fins  (as 
dorsal  and  anal)  supported  by  fin  rays,  and  whose  limbs 
(pectoral  and  ventral  fins)  do  not  exhibit  that  threefold 
division  (as  thigh,  leg,  and  foot)  found  in  most  other 
craniates. 

The  form  of  fishes  is  admirably  adapted  to  the  element 
in  which  they  live  and  move.  Indeed,  Nature  nowhere 
presents  in  one  class  such  elegance  of  proportions  with 
such  variety  of  form  and  beauty  of  color.  The  head  is 
disproportionately  large,  but  pointed  to  meet  the  resist- 
ance of  the  water.  The  neck  is  wanting,  the  head  be- 
ing a  prolongation  of  the  trunk  (Fig.  320).  The  viscera 
are  closely  packed  near  the  head,  and  the  long,  tapering 
trunk  is  left  free  for  the  development  of  muscles  which 
are  to  move  the  tail — the  instrument  of  locomotion 
(Fig.  321).  The  biconcave  vertebrae,  with  intervening 
DODGE'S  GEN.  ZOOL.  —  10 


146      STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 

cavities  filled  with  elastic  gelatin,  are  designed  for 
rapid  and  versatile  movements.  The  body  is  either 
naked,  as  in  the  bullhead  (Ameiurus),  or  covered  with 
polished,  overlapping  scales,  as  in  the  perch.  Rarely, 
as  in  the  sturgeon,  it  is  defended  by  bony  plates,  or  by 


ABC  V^r       D 

FlG.  119. —  Scales  of  fishes:  A,  cycloid  scale  (Salmon);  B,  ctenoid  scale  (Perch);  C, 
placoid  scale  (Ray) ;  D,  ganoid  scales  (Amblypterus}  —  a,  upper  surface;  b,  under 
surface,  showing  articulating  processes. 

minute,  hard  spines,  as  in  the  shark.  Scales  with  smooth, 
circular  outline  are  called  cycloid ;  those  with  notched  or 
spiny  margins  are  ctenoid.  Enameled  scales  are  ganoid, 
and  those  with  a  sharp  spine,  like  those  of  the  shark, 
are  placoid. 

The  vertical  fins  (dorsal,  anal,  and  caudal)  are  peculiar 
to  fishes.     The  dorsal  vary  in  number,  from  one,  as  in 


FIG.  120. — Bluefish  (Pomatomus  saltatrix) 


the  herring,  to  three,  as  in  the  cod ;  and  the  first  dorsal 
may  be  soft,  as  in  the  trout,  or  spiny,  as  in  the  perch. 
If  the  dorsals  are  cut  off,  the  fish  reels  to  and  fro.  The 


VERTEBRATA  147 

caudal  may  be  homocercal,  as  in  ordinary  species ;  or 
heterocercal,  as  in  sharks.  In  ancient  heterocercal 
fishes,  the  tail  was  frequently  vertebrated.  The  pectoral 
and  ventral  fins  stand  for  the  fore  and  hind  limbs  of 
other  vertebrates.  As  the  specific  gravity  of  the  body 
is  greater  than  that  of  the  water,  most  fishes  are  pro- 
vided with  an  air  bladder,  which  is  an  outgrowth  from 
the  esophagus.  This  is  absent  in  such  as  grovel  at  the 
bottom,  as  the  rays,  and  in  those,  like  the  sharks,  en- 
dowed with  compensating  muscular  power. 

Fishes  have  no  prehensile  organ  besides  the  mouth. 
Both  jaws  are  movable.     The  teeth  are  numerous,  and 


FIG.  121.  —  Salmon  (Salmo  salar).     Both  hemispheres. 

may  be  recurved  spines,  as  in  the  pike ;  flat  and  triangu- 
lar, with  serrated  edges,  in  the  shark ;  or  flat  and  tessel- 
lated in  the  ray  (Fig.  230).  They  feed  principally  on 
animal  matter.  The  digestive  tract  is  relatively  shorter 
than  in  other  vertebrates.  The  blood  is  red,  and  the 
heart  has  rarely  more  than  two  cavities,  an  auricle  and 
a  ventricle,  both  on  the  venous  side.  Ordinary  fishes 
have  four  gills,  which  are  covered  by  the  operculum,  and 
the  water  escapes  from  an  opening  behind  this.  In 
sharks  there  is  no  operculum,  and  each  gill  pouch  opens 
separately.  The  brain  consists  of  several  ganglia  placed 
one  behind  the  other,  and  occupies  but  a  small  part  of 
the  cranial  cavity  (Fig.  336).  Its  average  weight  to  the 


148      STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

rest  of  the  body  may  be  as  low  as  I  to  3000.  The  eggs 
of  bony  fishes  are  naked  and  multitudinous,  sometimes 
numbering  millions  in  a  single  spawn ;  those  of  the 
sharks  are  few,  and  protected  by  a  horny  shell  (Fig.  360). 
There  are  about  thirteen  thousand  species  of  fishes, 
of  which  over  two  thirds  are  Teleostomi.  There  are 
three  principal  subclasses  of  Pisces. 

SUBCLASS  I.  —  Elasmobranchii 

These  have  a  cartilaginous  skeleton,  and  a  skin  naked 
or  with  placoid  scales.  The  gill  openings  are  uncov- 
ered ;  and  the  mouth  is  generally  under  the  head.  The 


FIG.  122.  —  Shark  (Carcharias  vulgar  is).     Atlantic. 

ventral  fins  are  placed  far  back  ;  the  pectorals  are  large, 
in  the  rays  enormously  developed ;  and  the  tail  is  heter- 
ocercal.  Such  are  the  sharks,  rays,  and  dogfishes. 
They  are  all  marine.  The  largest  shark  found,  and 
therefore  the  largest  fish,  measured  forty  feet  in  length. 

SUBCLASS  II.  —  Teleostomi 

This  subclass  includes  all  of  the  common  fishes  having 
a  bony  endoskeleton  and  a  scaly  exoskeleton.  The 
skull  is  extremely  complicated ;  the  upper  and  lower 


VERTEBRATA  149 

jaws  are  complete,  and  the  gills  are  comblike  or  tufted, 
and  covered  by  an  operculum.  The  tail  is  homocercal 
except  in  the  "ganoids,"  as  the  sturgeon  and  garpike, 
in  which  it  is  heterocercal  or  unevenly  lobed ;  the  other 
fins  are  variable  in  number  and  position.  In  the  soft- 
finned  fishes,  the  ventrals  are  absent,  as  in  the  eels ;  or 
attached  to  the  abdomen,  as  in  the  salmons,  herrings, 


FIG.  123.  — Thornback  (Rafa  clavata).     European  seas. 

pikes,  and  carps;  or  placed  under  the  throat,  as  in  the 
cod,  haddock,  and  flounder.  In  the  spiny-finned  fishes, 
the  ventrals  are  generally  under  or  in  front  of  the  pec- 
torals, and  the  scales  ctenoid,  as  in  the  perches,  mullets, 
and  mackerels. 

The  so-called  "ganoids"  have  the  body  covered  with 
enameled  scales  or  bony  plates. 


150      STRUCTURAL  AND   SYSTEMATIC  ZOOLOGY 


FIG.  124.  —  Garpike  (Lepidosteus  osseus).     Lake  Ontario. 


FIG.  125.  —  Sturgeon  (Acipenser  sturio).     Atlantic  coast. 


FIG.  126.  —  Catfish,  or  Horned  Pout  (Ameiurus  nebulosus).    American  rivers. 


FIG.  127.  —  Cod  (Gadus  callarias}.    Atlantic  coast. 

SUBCLASS  III.  — Dipnoi 

These  fishes  connect  the  class  with  the  Amphibia. 
They  have  an  eel-like  body  sometimes  four  or  five  feet 
long,  covered  with  cycloid  scales ;  an  embryonic  noto- 
chord  for  a  backbone;  long,  ribbonlike  pectoral  and 
ventral  fins,  set  far  apart ;  two  incompletely  separated 
auricles,  and  one  ventricle ;  and,  besides  gills,  a  cellular 
air  bladder,  which  is  used  as  a  lung. 

They  live  in  muddy  or  stagnant  water  in  which  there 
is  little  oxygen  for  respiration,  not  enough  to  be  obtained 


VERTEBRATA  151 

by  the  gills  alone,  so  these  fishes  occasionally  come  to 
the  surface  and  take  air  into  the  lungs.  Lungfishes  feed 
upon  the  small  animals  captured  among  the  water  plants. 


FIG.  128.  —  Protopterus  annectens  ;  one  fourth  natural  size.     African  rivers. 

The  representatives  are  Ceratodus  from  Australia,  Pro- 
topterus from  Africa,  and  Lepidosiren  from  Brazil. 

CLASS  III.  —  Amphibia 

These  cold-blooded  vertebrates  are  distinguished  by 
having  gills  when  young,  and  usually  true  lungs  when 
adult.  They  have  no  fin  rays,  and  the  limbs,  when 
present,  have  the  same  divisions  as  those  of  higher  ani- 
mals. The  skin  is  soft,  and  generally  naked,  and  the 
skeleton  is  ossified.  The  skull  is  flat,  and  articulates 
with  the  spinal  column  by  two  condyles.  There  is  no 
distinct  neck ;  and  the  ribs  are  usually  small  or  wanting 
(Fig.  284).  The  heart  consists  of  two  auricles  and  one 
ventricle  (Fig.  273).  In  the  course  of  development 
nearly  all  undergo  metamorphosis  upon  leaving  the  egg, 
passing  through  the  "tadpole  "  state  (Fig.  370).  They 
commence  as  water-breathing  larvae,  when  they  resemble 
fishes  in  their  respiration,  circulation,  and  locomotion. 
In  the  lowest  forms,  the  gills  are  retained  through  life ; 
but  all  others  have,  when  mature,  lungs  only  (Fig.  282), 
the  gills  disappearing.  The  cuticle  is  frequently  shed, 
the  mode  varying  with  the  habits  of  the  species.34  The 
common  frog,  the  type  of  this  class,  stands  intermediate 


152      STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 


FIG.  129. —Head  and  gills  of  Necturus.  Cayuga 
Lake.  Copyright,  1901,  by  N.  Y.  Zoological 
Society. 


between  the  two  extremes  of  the  vertebrate  series;  no 
fundamental  part  is  excessively  developed. 

There  are  about  seven  hundred  living  species,  grouped 
in  three  orders :  — 

1 .  Urodela,  characterized  by  retaining  the  tail  through- 
out life,  and  in  usually  having  two  pairs  of  limbs  approx- 
imately equal  in  size. 
In    this    group    are 
the  Proteus  of  Aus- 
tria and  Necturus  of 
the  Eastern  United 
States,  both  of  which 
retain     their     gills ; 
Amphiuma  of  North 
America,    and     the 
salamanders        and 

newts,  in  which  the  gills  are  lost  in  the  adult,  though 
the  former  retains  a  gill  slit  as  an  evidence  of  their 
presence  in  the  larval  stage.35 

2.  Anura   include   all    the    well-known    amphibians 
which  are  tailless  in  the  adult  stage,  as  frogs  and  toads. 
They  have  a  moist, 

naked  skin,  ten  ver- 
tebrae, and  no  ribs. 
They  breath  by 
swallowing  the  air. 
They  have  four 
limbs  —  the  hinder 
the  longer,  and  the  first  developed.  They  have  four 
fingers  and  five  toes.  The  tongue  is  long,  and,  fixed 
by  its  anterior  end,  it  can  be  rapidly  thrown  out  as  an 
organ  of  prehension.36  The  eggs  are  laid  in  the  water 
enveloped  in  a  glairy  mass  ;  and  the  tadpoles  resemble 
the  urodelans  till  both  gills  and  tail  are  absorbed,  no 
gill  slit  persisting.  Frogs  (Rand)  have  teeth  in  the 


FIG.  130.  —  Red  Salamander  ( Spelerpes  ruber). 
United  States. 


VERTEBRATA 


153 


upper  jaw,  and  webbed  feet ;  toads  (Biifo)  are  higher  in 
rank,  and  have  neither  teeth    nor    fully  webbed  feet. 


FIG.  131.  —  Bullfrog  (Rana).     North  America. 

The  former  have  been  known  to  live  sixteen  years,  and 
the  latter  thirty-six. 


FIG.  132.  —  Proteus  anguinus.     Europe. 


3.    Gymnophiona  have  neither  tail  nor  limbs  nor  gill 
slit,  a  snakelike  form,  minute  scales  in  the  skin,  and  well- 


154      STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 

developed  ribs.     They  are  confined  to  the  tropics,  and 
are  subterranean  in  habit. 

CLASS  IV.  — Reptilia 

These  air-breathing,  cold-blooded  vertebrates  are  dis- 
tinguished from  all  fishes  and  amphibians  by  never  hav- 
ing gills,  and  from  birds  by  being  covered  with  horny 
scales  or  bony  plates.  The  skeleton  is  never  cartilag- 
inous (Fig.  310);  and  the  skull  has  one  occipital  condyle. 
The  vertebrae  are  ordinarily  concave  in  front;  and  the 
ribs  are  well  developed.  With  few  exceptions,  all  are 
carnivorous  ;  and  teeth  are  generally  present  (Figs.  231, 
235),  except  in  the  turtles,  where  a  horny  sheath  covers 
the  jaws.  The  teeth  are  never  fastened  in  sockets,  ex- 
cept in  crocodiles  (Fig.  224).  The  jaws  are  usually  very 
wide.  The  heart  has  three  chambers  (Fig.  273),  save  in 
crocodiles,  where  the  ventricle  is  partially  partitioned. 
But  in  all  cases  a  mixture  of  arterial  and  venous  blood  is 
circulated.  The  lungs  are  large,  and  coarsely  cellular 
(Fig.  281).  The  limbs,  when  present,  are  provided  with 
three  or  more  fingers  as  well  as  toes. 

There  are  about  three  thousand  species  of  living  rep- 
tiles, and  of  these  there  are  three  main  orders  :  the  first 
has  horny  scales,  the  others  have  bony  plates  combined 
with  scales. 

i.  Squamata,  including  the  lizards  and  the  snakes. 
The  lizards  (Lacertilia)  may  be  likened  to  snakes  provided 
with  fourlimbs,  each  having  five  digits.37  The  body  is  cov- 
ered with  horny  scales.  All  have  teeth,  which  are  simple 
in  structure;  and  the  halves  of  the  lower  jaw  are  firmly 
united  in  front,  while  those  of  snakes  are  loosely  tied 
together  by  ligaments.  Nearly  all  have  a  breastbone, 
and  the  eyes  (save  in  the  gecko)  are  furnished  with 
movable  lids.  In  the  common  lizards  and  chameleon, 
the  tongue  is  extensile.  The  tail  is  usually  long,  and  in 


VERTEBRATA 


155 


some  cases  each  caudal  vertebra  has  a  division  in  the 
middle,  so  that  the  tail,  when  grasped,  breaks  off  at  one 
of  these  divisions.  The  chameleon  has  a  prehensile  tail. 
The  iguana  is  distinguished  by  a  dewlap  on  the  throat 
and  a  crest  on  the  back.  Except  some  of  the  monitors 
of  the  Old  World,  all  the  lizards  are  terrestrial. 


f  FIG.  133.  —  Lizard  (Lacerta). 

The  snakes  (Ophidia)  are  characterized  by  the  absence 
of  visible  limbs ; 38  by  the  great  number  of  vertebrae, 
amounting  to  over  four  hundred  in  the  great  serpents ; 
by  a  corresponding  number  of  ribs,  but  no  sternum  ;  and 
no  true  eyelids,  the  eyes  being  covered  with  a  transparent 
skin.  The  tongue  differs  from  that  of  nearly  all  other 
reptiles  in  being  bifid  and  extensile.  The  mouth  is  very 
dilatable.  The  skin  is  frequently  shed,  and  always  by 
reversing  it.  Snakes  make  their  way  on  land  or  in  water 
with  equal  facility. 

As  a  rule,  the  venomous  snakes,  as  vipers  and  rattle- 
snakes, are  distinguished  by  a  triangular  head  covered 


156     STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 

with  small  scales ;  a  constriction  behind  the  head ;  two 
or  more  fangs,  and  few  teeth  ;  small  eyes,  with  vertical 


FIG.  134.  —  Adder,  or  viper  ( Vipera  berus}.      England. 

pupil;  and  short,  thick  tail.  In  the  harmless  snakes, 
the  head  gradually  blends  with  the  neck,  and  is  cov- 
ered with  plates ;.  the  teeth  are  comparatively  numerous 


FIG.  135.  —  a,  Head  of  a  harmless  snake  (upper  view");  b,  heads  of  various  venomous 

snakes. 

in  both  jaws ;  the  pupil  is  round,  and  the  tail  tapering. 
This  rule,  however,  has  many  exceptions. 


VERTEBRATA 


157 


2.  Chelonia,  or  tortoises  and  turtles,  are  of  anomalous 
structure.  The  skeleton  is  external,  so  as  to  include  not 
only  all  the  viscera,  but  also  the  whole  muscular  system, 
which  is  attached  internally ;  and  even  the  limbs  are 


FIG.  136.  —  Hawkbill  turtle  (Chelone  imbricata}.     Tropical  Atlantic. 

inside,  instead  of  outside,  the  thorax.  The  exoskeleton 
unites  with  the  endoskeleton,  forming  the  carapace,  or 
case,  in  which  the  body  is  inclosed.  The  exoskeleton 
consists  of  horny  plates,  known  as  "  tortoise  shell "  (in 

the  soft  tortoises, 
Aspidonectes,  this 
is  wanting),  and 
of  dermal  bones, 
united  to  the  ex- 
panded spines  of 
the  vertebrae  and 
to  the  ribs,  making 
the  walls  of  the 
carapace(Fig.3i2). 
The  ventral  pieces  form  the  plastron.2®  All  are  tooth- 
less. There  are  always  four  stout  legs ;  and  the  order 
furnishes  the  only  examples  of  vertebrates  lower  than 
birds  that  really  walk,  for  lizards  and  crocodiles  wriggle, 


FIG.  137.  —  Box  tortoise  (Terrapene  Carolina}. 
United  States. 


158      STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

and  drag  the  body  along.  There  are  no  teeth,  but  a 
horny  beak.  The  eggs  are  covered  with  a  calcareous 
shell. 

The  sea  turtles,  as  the  edible  green  turtle  and  the 
hawkbill  turtle,  which  furnish  the  "tortoise  shell"  of 
commerce,  have  the  limbs  converted  into  paddles.  The 
fresh-water  forms,  represented  by  the  snapping  turtle 
(Chelydra),  are  amphibious,  and  have  palmated  feet. 
Land  tortoises  (Testudo)  have  short,  clumsy  limbs, 
fitted  for  slow  motion  on  the  land ;  the  plastron  is  very 
broad,  and  the  carapace  is  arched  (while  it  is  flattened 
in  the  aquatic  species),  and  head,  legs,  and  tail  can  be 
drawn  within  it.  The  land  and  marine  species  are 
vegetable  feeders ;  the  others,  carnivorous. 

3.  Crocodilia,  the  highest  and  largest  of  reptiles,  have 
also  two  exoskeletons  —  one  of  horny  scales  (epidermal), 


FIG.  138. —  Alligator  (A.  misszssz^zensz's) .     Southern  States. 


and  another  of  bony  plates  (dermal).  The  bones  of  the 
skull  are  firmly  united,  and  furnished  with  numerous 
teeth,  implanted  in  distinct  sockets.  The  lower  jaw 
extends  back  of  the  cranium.  The  heart  has  four 
cavities,  but  the  pulmonary  artery  and  aorta  commu- 
nicate with  each  other,  so  that  there  is  a  mixture  of 
venous  and  arterial  blood.  They  have  external  ear 
openings,  closed  by  a  flap  of  the  skin,  and  eyes  with 
movable  lids ;  a  muscular  gizzard ;  a  long,  compressed 


VERTEBRATA  159 

tail ;  and  four  legs,  with  feet  more  or  less  webbed,  and 
having  five  toes  in  front  and  four  behind.  The  existing 
species  are  confined  to  tropical  rivers,  and  are  carnivo- 
rous. The  eggs  are  covered  with  a  hard  shell. 

There  are  three  representative  forms:  the  gavial  of 
the  Ganges,  remarkable  for  its  long  snout  and  uniform 
teeth ;  the  crocodiles,  mainly  of  the  Old  World,  whose 
teeth  are  unequal,  and  the  lower  canines  fit  into  a  notch 
in  the  edge  of  the  upper  jaw,  so  that  it  is  visible  when 
the  mouth  is  closed ;  and  the  alligators  of  the  New 
World,  whose  canines,  in  shutting  the  mouth,  are 
concealed  in  a  pit  in  the  upper  jaw.  The  toes  of  the 
gavials  and  crocodiles  are  webbed  to  the  tip ;  those  of 
the  alligators  are  not  more  than  half  webbed. 

In  the  mediaeval  ages  of  geological  history,  the  class 
of  reptiles  was  far  more  abundantly  represented  than 
now.  Among  the  many  forms  which  geologists  have 
unearthed  are  numerous  gigantic  saurians,  which  can- 
not be  classified  with  any  of  the  four  living  orders. 
Such  are  the  Ichthyosaurus,  Plesiosaurus,  Pterodactylus, 
Megalosaurus,  and  Iguanodon. 

CLASS  V.  —  Aves 

Birds  form  the  most  clearly  defined  class  in  the  whole 
animal  kingdom,  and  in  some  respects  are  the  most 
highly  specialized  of  the  craniata.  The  eagle  and  hum- 
mer, the  ostrich  and  duck,  widely  as  they  seem  to  be 
separated  by  size,  form,  and  habits,  still  exhibit  one 
common  type  of  structure.  On  the  whole,  birds  are 
more  closely  allied  to  reptiles  than  to  mammals.  In 
number,  they  approach  the  fishes,  ornithologists  having 
determined  eight  thousand  species,  or  more. 

A  bird  is  an  air-breathing,  egg-laying,  warm-blooded, 
feathered  vertebrate,  with  two  limbs  (legs)  for  perching, 
walking,  or  swimming,  and  two  limbs  (wings)  for  flying 


160      STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 

or  swimming.  Organized  for  flight,  it  is  gifted  with  a 
light  skeleton,  very  contractile  muscular  fibers,  and  a 
respiratory  system  of  the  highest  development. 

The  skeleton  is  more  compact  than  those  of  reptiles 
and  mammals,  at  the  same  time  that  it  is  lighter,  and 
the  bones  are  harder  and  whiter.  It  contains  fewer 
bones  than  usual,  many  parts  being  anchylosed  together, 
as  the  skull  bones,  the  dorsal  vertebrae,  and  bones  of  the 
tarsus  and  metatarsus.  The  lumbar  vertebrae  are  united 
to  the  ilia.  The  neck  is  remarkably  long  (containing 
from  nine  to  twenty-four  vertebrae),  and  flexible,  ena- 
bling the  head  to  be  a  most  perfect  prehensile  organ. 
The  ribs  are  generally  jointed  in  the  middle,  as  well  as 
with  the  backbone  and  sternum.  The  last,  where  the 
muscles  of  flight  originate,  is  highly  developed.  The 
skull  articulates  with  the  spinal  column  by  a  single 
condyle,  and  with  the  lower  jaw,  not  directly,  as  in 
mammals,  but  through  the  intervention  of  a  separate 
bone,  as  in  reptiles  (Fig.  313). 

All  birds  have  four  limbs,  while  every  other  verte- 
brate class  shows  exceptions.  The  fore  limbs  are  fitted 
for  flight.  They  ordinarily  consist  of  nine  separate 
bones,  and  from  the  hand,  fore  arm,  and  humerus  are 
developed  the  primary,  secondary,  and  tertiary  feathers 
of  the  wing.  The  hind  limbs  are  formed  for  progres- 
sion—  walking,  hopping,  running,  paddling,  and  also 
for  perching  and  grasping.  The  modifications  are  more 
numerous  and  important  than  those  of  the  bill,  wing,  or 
tail.  There  are  twenty  bones  ordinarily,  of  which  the 
tibia  is  the  principal ;  but  the  most  characteristic  is  the 
tarsometatarsus,  which  is  a  fusion  of  the  lower  part  of 
the  tarsus  with  the  metatarsus.  The  rest  of  the  tarsus 
is  fused  with  the  tibia.  The  thigh  is  so  short  that  the 
knee  is  never  seen  outside  of  the  plumage;  the  first 
joint  visible  is  the  heel.40  Most  birds  have  four  toes 


VERTEBRATA 


161 


(the  external  or  "  little  "  toe  is  always  wanting) ;  many 
have  three,  the  hallux,  or  "big  "  toe,  being  absent ;  while 
the  ostrich  has  but  two,  answering  to  the  third  and 
fourth.  The  normal  number  of  phalanges,  reckoning 
from  the  hallux,  is  2,  3,  4,  5.  The  toes  always  end  in 
claws. 

Birds  have  neither  lips  nor  teeth,  epiglottis  nor  dia- 
phragm. The  teeth  are  wanting,  because  a  heavy  mas- 
ticating apparatus  in 
the  head  would  be 
unsuitable  for  flight. 
The  beak,  crop,  and 
gizzard  vary  with  the 
food.  It  is  a  pecul- 
iarity of  all  birds, 
though  not  confined 
to  them,  that  the  gen- 
erative products  and 
the  refuse  of  digestion 
are  all  discharged 
through  one  common 
outlet. 

The  sole  organs  of 
prehension  are  the 
beak  and  feet.  The 
circulation  is  double, 
as  in  mammals,  start- 
ing from  a  four- 
chambered  heart  (Fig.  273).  Respiration  is  more  com- 
plete than  in  other  vertebrates.  The  lungs  are  fixed,  and 
communicate  with  air  sacs  in  various  parts  of  the  body,  as 
along  the  vertebral  column,  and  also  with  the  interior  of 
many  bones,  as  the  humerus  and  femur,  which  are  usually 
hollow  and  marrowless.41  Both  brain  and  cord  are  much 
larger  relatively  than  in  reptiles  (Fig.  338);  the  cranium 
DODGE'S  GEN.  ZOOL. — ,11 


deb 


FIG.  139.  —  Principal  parts  of  a  bird:  a,  primaries; 
6,  secondaries;  c,  spurious  wing;  d,  wing  coverts; 
e,  tertiaries;  _/,  throat,  or  jugulum;  g,  chin;  k,  bill; 
the  meeting  line  between  the  two  mandibles  is  the 
commissure;  the  ridge  on  the  upper  mandible  is 
called  culmen;  that  of  the  lower,  gonys;  the  space 
between  the  base  of  the  upper  mandible  and  the 
eye  is  the  lore;  /*,  forehead;  k,  crown;  /,  scapular 
feathers;  m,  back;  n,  metatarsus,  often  called  tarsus 
or  tarsometatarsus;  o,  abdomen;  p,  rump;  q,  upper 
tail  coverts;  r,  lower  tail  coverts. 


162      STRUCTURAL  AND    SYSTEMATIC  ZOOLOGY 


is  larger  in  proportion  to  the  face ;  and  the  parts  of  the 
brain  are  not  situated  in  one  plane,  one  behind  the  other. 
The  cerebrum  is  round  and  smooth,  and  the  cerebellum 
single-lobed.  The  ears  resemble  those  of  crocodiles  ;  but 
the  eyes  are  well  developed,  and  protected  by  three  lids. 
They  are  placed  on  the  sides  of  the  head,  and  the  pupil 
is  always  round.  The  sexes  generally  differ  greatly  in 
plumage,  in  some  cases  more  widely  than  two,  distinct 
species,  but  the  coloration  of  either  sex  of  any  one 
species  is  very  constant. 

There  are  two  divisions  of  living  birds. 

DIVISION  A.  —  Ratitae  (jCursores) 

This  small   and    singular  group  is  characterized   by 
having  no  keel  on  the  breastbone,  rudimentary  wings, 

feathers  with  discon- 
nected barbs,  and 
stout  legs.  The  Af- 
rican ostrich  has  two 
toes,  the  cassowary 
three,  and  the  apte- 
ryx  four. 

Its  representatives 
are  the  ostrich(5/r//- 
thid)  of  Africa  and 
Arabia,  South  Amer- 
ican ostrich  (Rhea\ 
cassowary  ( Casua- 
rins)  of  the  East 
Indian  Archipelago 
and  Australia,  emu 
(Drom&us)  of  Aus- 
tralia, and  Apteryx, 

FIG.  140.  —  African  Ostrich  (Struthio  camelus). 


or  kiwikiwi     of 

Zealand.    Besides  these,  there  are  extinct  gigantic  forms 


VERTEBRATA  163 

from  Madagascar  (sEpyornis)  and  from  New  Zealand, 
the  moa  (Dinornis).  This  singular  geographical  distri- 
bution, like  that  of  the  Dipnoi  and  marsupials,  shows 
that  the  group  was  once  widely  spread  over  the  earth, 
but  is  now  greatly  restricted  in  area. 

DIVISION  B.  —  Carinatae 

Birds  which,  with  rare  exceptions,  e.g.,  the  Penguins, 
have  a  keeled  sternum,  and  developed  functional  wings. 

Of  the  birds  composing  this  division,  some  live  mainly 
in  the  water,  others  on  the  land,  while  still  others  spend 
a  considerable-  part  of  their  lives  on  the  wing.  Their 
bodily  structure  is,  consequently,  modified  to  suit  their 
mode  of  life.  Hence,  they  may  be  broadly  grouped 
into  aquatic,  terrestrial,  and  aerial  birds. 

A.  AQUATIC  BIRDS.  —  Specially  organized  for  swim- 
ming ;  the  body  flattened,  and  covered  with  water-proof 
clothing  —  feathers  and  down  ;  the  legs  short  (the  knees 
being  wholly  withdrawn  within  the  skin  of  the  body), 
and  set  far  apart  and  far  back ;  the  feet  webbed,  and 
hind  toe  elevated  or  absent.     The  legs  are  always  feath- 
ered to  the  heel  at  least.    They  are  the  only  birds  whose 
neck  is  sometimes  longer  than  the  legs. 

Examples,  penguins,  ducks,  petrels,  and  gulls. 

B.  TERRESTRIAL  BIRDS.  —  This  group  exhibits  great 
diversity  of  structure ;  but  all  agree  in  being  especially 
terrestrial  in  habit,  spending  most  of  the  time  on  the 
ground,  not  on  trees  or  the  water,  although   many  of 
them  fly  and  swim  well.     The  legs  are  long  or  strong, 
and  the  knee  is  free  from  the  body.     The  hind  toe,  when 
present,  is  small  and  elevated.    Such  birds  are  the  storks, 
plovers,  and  turkeys. 

C.  AERIAL  BIRDS.  —  This  highest  and  largest  group 
includes  all  those  birds  whose  toes  are  fitted  for  grasping 
or  perching,  the  hind  toe  being  on  a  level  with  the  rest. 


164      STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

The  knee  is  free  from  the  body,  and  the  leg  is  generally 
feathered  to  the  heel.  The  wings  are  adapted  for  rapid 
or  long  flight,  and  they  hop,  rather  than  walk,  on  the 


FIG.  141.  —  Loon  (Urinator  imber).     North  America. 

ground.42      They  always  live  in  pairs,  and  the  young 
are  hatched  helpless.     In  this  group  may  be  placed  the 

pigeons,  birds   of  prey,  par- 
rots, and  the  song  birds. 

The  more  important  orders 
of  birds  are  the  following  :  — 
i.  Pygopodes,  or  divers. 
These  lowest  of  the  feath- 
ered tribe  have  very  short 
wings  and  tail,  and  the  legs 
are  placed  so  far  back  that 
they  are  obliged,  when  on 
land,  to  stand  nearly  bolt 
upright.  They  are  better 
fitted  for  diving  than  for 
flight,  or  even  swimming. 

FIG.  142.—  Penguin  (Aptenodytes  pen-    They      belong      tO      the      high 
nantii).     Falkland  Islands.  ,       .        ,  ...  _    . 

latitudes,     living     on     fishes 
mainly,  and  are  represented  by  the  loons  and  grebes. 


VERTEBRATA 


I65 


2.  Impennes,  or  penguins.      These  birds,  found  only 
in  the  southern  hemisphere,  have  many  of  the  structural 
features  of  those  in  the  preceding  order,  but  their  wings 
are  so  rudimentary  that  flight  is  impossible  (Fig.  142). 

3.  Turbinares,  the  albatrosses  and  petrels  (largest  and 
smallest  of  web-footed  birds),  having  a  hawklike,  hooked 
bill  and  the  nostrils  opening  through  tubes.     The  wan- 
dering ^albatross  inhabits  the  southern  seas  but  some- 
times comes  as  far  north  as  Florida.     It  measures  twelve 
to  fourteen  feet  from  tip  to  tip  of  its  wings.     Wilson's 
petrel  (Oceanites\  one  of   the   smallest   of   the   many 
species,    is   known   to   the   sailor  as   "  Mother  Carey's 
chicken."     The  birds 

in  this  order  are  noted 
for  their  powers  of 
flight. 

4.  Stega no p odes, 
characterized     by     a 
long    bill,    generally 
hooked ;  wings  rather 
long ;   and  toes  long, 
and  all  four  joined  to- 
gether by  broad  webs. 
Throat  generally  na- 
ked,   and    furnished 
with  a  sac.     The  ma- 
jority are   large  sea- 
birds,      and      feed      On    FIG-    143- —  Cormorant    (Phalacrocorax).       Copy- 

right,  1901,  by  N.  Y.  Zoological  Society. 

fishes,  mollusks,  and 

insects.     Examples  are  the  cormorants,  pelicans,  gan- 

nets,  and  frigate  bird  (Fig.  143). 

5.  Herodiones.     The  herons,  bitterns,   storks,  ibises, 
spoonbills  and  flamingoes   are  included  in    this  order. 
(Fig.    144).      They  are   readily  distinguished   by  their 
long  and  bare  legs.     Generally,  also,  the  toes,  neck,  and 


IP*  • 


166      STRUCTURAL  AND   SYSTEMATIC  ZOOLOGY 

bill   are   of   proportionate   length,-  and  the   tail   short. 
They  feed  on  small  animals,  and,  with  a  few  exceptions, 


FIG.  144.  —  Heron  (Ardea). 

frequent  the  banks  of  rivers.  In  flying,  their  legs  are 
stretched  out  behind,  while  in  most  other  birds  they 
are  folded  under  the  body.  • 

6.  Anseres  have  a  heavy  body,  moderate  wings,  short 
tail,  flattened  bill,  covered  by  a  soft  skin,  with  ridges 
along  the  edges.     Diet  more  commonly  vegetarian  than 
animal.      The    majority  inhabit   fresh   water  —  as   the 
ducks,  geese,  and  swans.  •"'-•'.  !~ 

7.  Accipitres,  including   the   diurnal   birds   of   prey. 
They  have  a  strongly  hooked  beak  with  a  waxy  mem- 
brane (cere)  at  the  base  of  the  upper  mandible,  three 
toes  in  front  and  one  behind.     The  toes  are  armed  with 
long,  strong,  crooked  talons;  the  legs  are   robust,  the 
tarsus  and  toes  usually  without  feathers  being  covered 
by   scales ;    and   the   wings   are    of   considerable   size, 


VERTEBRATA 


I67 


FlG.  145.  —  Wild  duck  (Anas  boschas}.     North  America. 


FIG.  146.  —  Wild  geese   (Branta  canadensis).     United  States.     Copyright,  1901,  by 
N.Y.  Zoological  Society. 


1 68      STRUCTURAL  AND   SYSTEMATIC  ZOOLOGY 

adapted  for  rapid  and  powerful  flight.     The  bill  is  stout 
and  sharp,  and  usually  toothed.    The  eyes  are  on  the  sides 


FIG.  147.  —  Fishhawk  (Pandion  haliaetus  carolinensis}.     United  States. 

of  the  head,  the  wings  pointed,  and  the  plumage  firm  and 
close.    All  are  carnivorous.    The  female  is  larger  than  the 

male,  except  the  con- 
dor. -  Examples  are  the 
eagles,  hawks,f  alcons, 
kites,  and  vultures. 

8.  Gallince.  As  a 
rule,  this  order,  so 
valuable  to  man,  is 
characterized  by  a 
short,  arched  bill  ; 
short  and  concave 
wings,  unfitted  for 
protracted  fl  i  g  h  t  ; 
stout  legs,  of  medium 
length;  and  four  toes, 

FlG.  148.  —  Golden  eagle   (Aquila  chrysaetos). 


North  America  and  Europe.  Copyright,  1901 , 
by  N.Y.  Zoological  Society. 


the    three    in 
being     united 


front 
by    a 


VERTEBRATA 


169 


short  web,  and  terminating  in  blunt  claws.     The  legs 

are  usually   feathered   to   the    heel,    sometimes   (as   in 

grouse)   to   the  toes. 

The   feathers   of  the 

body    are    large    and 

coarse.      The    males 

generally    have     gay 

plumage,    and    some 

appendage      to      the 

head.      The    nostrils 

are  covered  by  a  scale 

or  valve.    Their  main 

food  is  grain.      Such 

are     the     partridges, 

turkeys,       pheasants, 

and   poultry. 

9.  Gralla.  The  rails 

and  cranes  are  long- 
legged,    marsh    birds 

with  four  toes,  of  which  the  hinder  one  is  usually  small 

and  higher  up  than 
the  front  ones.  The 
feet  are  adapted  for 
wading,  for  standing 
upon  floating  vegeta- 
tion, or  walking  over 
soft  mud,  having  long 
spreading  toes  which 
aid  in  distributing 
the  weight  of  the 
body  over  much  sur- 
face. Cranes  eat 


FIG.  149. — Prairie  chicken  (Cupidonia  cupido), 
Western  prairies. 


snakes  as  well  as  veg- 
etable food,  while  rails  are  fond  of  mollusks  and  worms. 


I/O      STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 


10.  Gavice,  distinguished  by  their  long,  pointed 
wings,  usually  long  tail,  and  by  great  powers  of  flight. 
They  are  all  carnivorous.  Such  are  the  gulls  and  terns, 
which  frequent  the  seacoast,  lakes,  and  rivers;  and  the 


FIG.  151.— Tern  (Sterna), 

auk  which  is  found  in 'northern  seas.     The  great  auk 

was  flightless  and  became  extinct  as  a  result  of  unre- 
stricted killing  by 
fishermen  and  oth- 
ers who  regularly 
visited  the  nesting 
grounds  of  this  bird 
on  the  islands  and 
along  the  coast  of 
the  North  Atlantic 
for  the  purpose 
of  collecting  the 
eggs,  and  killing 
the  birds  for  their 
feathers  and  oil. 

The  last  living  specimen  of  the  great  auk  was  seen  in  1 842. 
ii.  Limicolce,  or  shore  birds,  include  the  snipes, 

plovers,  woodcock,  sandpipers,  phalaropes,  stilts,  avocets, 


FIG.  152.  —  Sandpiper  ( T. 


hypoleuca) .      England. 


VERTEBRATA 


171 


-Ve 


and  jacanas.     The  toes  are  three  or  four  in    number, 

with  the    hind   one  when   present  elevated   above  the 

others,  the  legs  are  long, 

slender,     and     bare     be- 
low.   The  phalaropes  have 

webbed  toes  and  can  swim. 

They    feed    mainly    upon 

worms     and      crustaceans 

which  they  dig  out  of  the 

mud  or  from  under  stones 

with  their  long  bills. 

12.    Columbce,  or  pigeons 

and  doves,  have  wings  for 

prolonged  flight,  and  slen- 
der legs,  fitted  rather  for  an 

arboreal  life,  with  toes  not 

united,  and  the  hind  toe  on  FIG.  i53.  — Ringdove 

a  level  with  the  rest.  Their 

mode  of  drinking  is  peculiar,  the  head  not  being  raised 

when  the  water 
is  swallowed. 
The  passenger 
pigeon,  which 
was  formerly 
very  abundant 
in  some  sections 
of  the  country, 
seems  to  be 
approaching  ex- 
tinction. Wilson, 
the  ornitholo- 
gist, saw  a  flock 

FIG.  154.  —  Foot  of  parrot  and  woodpecker.  jj^    I  808    in    JCen- 

tucky  in  which  he  estimated  there  were  2,230,272,000 
individuals.     At    present    the   bird  is   seen  only  occa- 


1/2 


STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 


sionally.  The  extinct, 
flightless  dodo  (Didus), 
a  native  of  the  island 
of  Mauritius,  belonged 
'in  this  order. 

13.  Ps  it  tad,  or  par- 
rots. These  birds  have 
a  strong,  arched  upper 


FIG.  155.  —  Barn  owl  (Stn'x  pratin- 

cola).      Both     hemispheres.     Copy-  Jr^i 

right,    1901,   by   N.   Y.    Zoological  "\'^-<m 

Society.  f*«S 


bill,  with  a  cere  at  the 
base,  a  fleshy,  thick,  and 
movable  tongue,  and 
paired  toes.  They  have, 
usually,  brilliant  plu- 
mage. They  live  in  trees 
and  feed  on  fruits.  Such' 
are  the  parrots,  paro- 
quets, cockatoos,  and 
macaws. 


FIG.  156.  —  Trogon  elegans.     Central  America. 


VERTEBRATA  173 

14.  Striges,  or  owls,  have  many  of  the  characters  of 
the  Accipitres,  but  may  be  distinguished  from  them  by 
having  their  large  eyes  directed  forward  and  surrounded 
by  radiating  feathers,  a  feathered  tarsus,  and  soft  plu- 
mage (Fig.  155). 

15.  Picari&.    This  heterogeneous  group  is  sometimes 
subdivided  into  several  orders  since   its   members    are 
too  unlike,  structurally,  to  be  classed  together.43     The 
toes  are  usually  paired,  two  in  front  and  two  behind. 


FIG.  157.  —Goatsucker  {Caprimulgus). 

As  here  given  the  order  includes  swifts,  goatsuckers 
(Fig.  157),  humming  birds,  cuckoos,  kingfishers  (Fig. 
158),  trogons  (Fig.  156),  toucans,  hornbills,  hoopoes, 
and  woodpeckers.  These  birds  are  not  musical,  and  only 
ordinary  fliers.  The  most  of  them  feed  on  insects  or  fruit. 
The  majority  make  nests  in  the  hollows  of  old  trees;1 
but  the  cuckoos  often  lay  in  the  nests  of  other  birds. 
In  climbing,  the  woodpeckers  are  assisted  by  their  stiff 
tail. 


1/4      STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


FIG.  158.  —  Kingfisher  (Ceryle). 


FIG.  159.  —  Head  of  a  flycatcher  ( Tyrannus). 


VERTEBRATA  175 

1 6.    Passeres.    This  order  is  the  most  numerous  and 
varied  in  the  whole  class.     It  comprehends   all  those 


FIG.  160.  —  White-throated  sparrow  (Zonotrichia  albicollis).    United  States. 

tribes  which  live  habitually  among  trees,  excepting  the 
rapacious  and   climbing  birds,  whose   toes  —  three  in 


FIG.  161.  —  Redstart  (Setophaga  ruticitta).    United  States. 

front,  and  one  behind  —  are  eminently  fitted  for  perch- 
ing only.  The  legs  are  slender,  and  seldom  used  for 
locomotion. 

They  are  divisible  into  two  sections :  a.  Clamatores, 
having  the  tarsus  usually  enveloped  in  a  row  of  plates 
meeting  behind  in  a  groove,  and  the  bill  broad,  and  bent 


1/6      STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

down   abruptly   at  the   tip.      Representatives   are   the 
tyrant  flycatchers  (Fig.  159).     b.  Oscines,  or  songsters, 


FIG.  162.  —  White  eyed  vireo  (Fz>v0  noveboracensis).     United  States. 

all  of  whom  have  a  vocal  apparatus,  though  not  all  sing. 
The  anterior  face  of  the  tarsus  is  one  continuous  plate, 
or  divided  transversely  into  large  scales ;  and  the  plates 
on  the  sides  meet  behind  in  a  ridge.  The  toes,  always 
three  in  front  and  one  behind,  are  on  the  same  level. 
The  eggs  are  usually  colored.  Here  belong  the 
crows,  jays,  birds  of  paradise,  blackbirds,  orioles,  larks, 


FIG.  163.  —  Swallow  (Hirundo). 


sparrows  (Fig.  160),  tanagers,  finches,  waxwings,  swal- 
lows (Fig.  163),  wrens,  warblers  (Figs.  161,  162), 
thrushes,  etc. 


VERTEBRATA 


177 


CLASS  VI.  — Mammalia 

Mammals  are  distinguished  from  all  other  vertebrates 
by  any  one  of  the  following  characters :  they  suckle 
their  young;  the  thorax  and  ab- 
domen are  separated  by  a  perfect 
diaphragm ;  the  red  corpuscles 
of  the  blood  have  no  nucleus, 
and  are  therefore  double  concave 
(Fig.  259),  and  either  a  part  or 
the  whole  of  the  body  is  hairy  at 
some  time  in  the  life  of  the  ani- 
mal (Fig.  291,  30 1  ).44 

They  are  all  warm-blooded  ver- 
tebrates, breathing  only  by  lungs, 
which  are  suspended  freely  in 
the  thoracic  cavity ;  the  heart  is 
four-chambered,  and  the  circula- 
tion is  double,  as  in  birds  (Fig. 
273);  the  aorta  is  single,  and 
bends  over  the  left  bronchial 
tube ;  the  large  veins  are  fur- 
nished with  valves ;  the  red  cor- 
puscles differ  from  those  of  all 
other  vertebrates  in  having  no 
nucleus  and  in  being  circular  (ex- 
cept in  the  camel) ;  the  entrance 
to  the  windpipe  is  always  guarded 
by  an  epiglottis  ;  the  cerebrum  is 
more  highly  developed  than  in 
any  other  class,  containing  a 
greater  amount  of  gray  matter 
and  (in  the  higher  orders)  more  convolutions ;  the  cere- 
bellum has  lateral  lobes,  a  mammalian  peculiarity,  and 
there  is  a  corpus  callosum  and  a  pons  varolii  (Fig.  335, 
DODGE'S  GEN.  ZOOL. —  12 


FIG.  164.  —  Longitudinal  section 
of  human  body  (theoretical) :  a, 
cerebro-spinal  nervous  system; 
b,  cavity  of  nose;  c,  cavity  of 
mouth;  d,  alimentary  canal; 
e,  chain  of  sympathetic  ganglia; 
/,  heart;  g,  diaphragm. 


1 78      STRUCTURAL   AND    SYSTEMATIC  ZOOLOGY 

339,  342),  the  cranial  bones  are  united  by  sutures,  and 
they  are  fewer  than  in  cold-blooded  vertebrates ;  the 
skull  has  two  occipital  condyles,  a  feature  shared  by 
the  amphibians ;  the  lower  jaw  consists  of  two  pieces 
only  (often  united),  and  articulates  directly  with  the 
cranium ;  with  four  exceptions  there  are  always  seven 
cervical  vertebrae : 45  the  dorsal  vertebrae,  and  therefore 
the  ribs,  vary  from  ten  to  twenty-four;  the  lumbar 
vertebrae  number  from  two  to  nine;  the  sacral  from 
three  to  nine,  and  the  caudal  from  two  to  forty-six ;  the 

articulating  surfaces  of  the  ver- 
tebrae are  generally  flat;  the 
fore  limbs  are  never  wanting, 
and  the  hind  limbs  only  in  a 
few  aquatic  forms;  excepting 
the  whales,  each  digit  carries 
a  nail,  claw,  or  hoof  ;  the  teeth 
(always  present,  save  in  certain 

bro-spinal   nervous  axis  contained       ]ow    tribes)    are    USUally  in    tWO 

in  neural  tube;  e,  chain  of  sympa-  '  J 

thetic  ganglia;  </,  alimentary  canal;       Sets,    a    milk    Or    deClduOUS    Set, 

and  a  permanent  set,  and  are 

planted  in  sockets ;  the  mouth  is  closed  by  flexible 
lips ;  an  external  ear  is  rarely  absent ; 46  the  eyes  are 
always  present  though  rudimentary  in  some  burrow- 
ing animals ;  they  are  viviparous ;  and,  finally,  and 
perhaps  above  all,  while  in  all  other  animals  the 
embryo  is  developed  from  the  nourishment  laid  up  in 
the  egg  itself,  in  mammals  it  draws  its  support,  almost 
from  the  beginning,  directly  from  the  parent,  and, 
after  birth,  it  is  sustained  for  a  time  by  the  milk  se- 
creted by  the  mammary  glands  (Fig.  367).  From 
the  first,  therefore,  till  it  can  care  for  itself,  the  young 
mammal  is  in  vital  connection  with  the  parent. 

About  twenty-one  hundred  species  are  known,  inhabit- 
ing the  land,  the  water,  and  the  air. 


VERTEBRATA  179 

SUBCLASS  I.  —  Prototheria 

These  mammals  have  but  one  outlet  for  the  intestine, 
urinary  and  reproductive  organs,  as  in  birds.  They  are 
implacental,  and  the  mammary  glands  are  rudimentary. 
There  is  but  one  order. 

i.  Monotremata,  This  order  includes  two  singular 
forms,  the  duck  mole  (Ornithorhynchus)  and  spiny 
ant-eater  (Echidna),  both  confined  to  the  Australian 


FIG.  166.  —  Ornithorhynchus. 

continent  and  New  Guinea.  The  former  has  a  cover- 
ing of  fur,  a  bill  like  that  of  a  duck,  and  webbed  feet. 
The  latter  is  covered  with  spines,  has  a  long,  toothless 
snout,  like  the  ant-eater's,  and  the  feet  are  not  webbed. 
Both  burrow,  and  feed  upon  insects.  The  brain  is 
smooth  in  the  ornithorhynchus,  and  folded  in  the 
echidna.  In  both,  the  cerebral  hemispheres  are  loosely 
united  by  transverse  fibers,  and  do  not  cover  the  cere- 
bellum and  olfactory  lobes,47  Both  lay  eggs  which 
resemble  those  of  birds  and  from  which  the  young  are 
hatched. 


ISO      STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

SUBCLASS  II.  —  Theria 

In  these  mammals  the  mammary  glands  are  typically 
developed  and  there  is  no  cloaca. 
There  are  two  sections  :  — 


SECTION  A.  —  Metatheria 

In  this  group  are  the  marsupials,  opossums,  bandi- 
coots, wombats,  and  kangaroos. 

Marsupialia  are  distinguished  by  the  fact  that  the 
young,  always  born  premature,  are  transferred  by  the 
mother  to  a  pouch  on  the  abdomen,  where  they  are 
attached  to  the  nipples,  and  the  milk  is  forced  into 


FIG.  167.  — Virginia  opossum  (Didelphys  virginianci). 

their  mouths  by  special  muscles.48  They  have  "  mar- 
supial bones"  projecting  from  the  pelvis,  which  may 
serve  to  support  the  pouch ;  though  as  the  monotremes 
have  the  same  bones,  but  no  pouch,  they  doubtless 
have  some  other  function.  These  bones  are  peculiar 
to  animals  having  no  placenta,  namely,  to  monotremes 
and  marsupials.  The  brains  of  marsupials  resemble 
those  of  the  monotremes,  except  that  the  cerebrum  of 


VERTEBRATA  l8l 

the  kangaroo  covers  the  olfactory  lobes.  All  have  the 
four  kinds  of  teeth,  and  all  are  covered  with  fur,  never 
with  spines  or  scales.  Except  the  opossums  of  Amer- 
ica, all  are  restricted  to  Australia  and  adjacent  islands. 
The  marsupials  are  almost  the  only  mammals  of 
Australia,  there  being  very  few  species  of  placental 
mammals.  The  marsupials  have  here  developed  into 
forms  corresponding  in  their  habits  to  the  orders  of 
placental  mammals  in  the  rest  of  the  world.  The 
kangaroos  take  the  place  of  the  large  herbivores  — 
the  ungulates.  The  thylacinus  and  dasyurus  are  the 
marsupial  carnivora.  Other  forms  are  squirrel-like  in 
shape  and  habits,  and  still  others  are  insectivorous. 

SECTION  B.  —  Eutheria 

In  these  mammals  the  young  are  connected  with  the 
mother  by  means  of  a  vascular  structure,  the  placenta, 
by  which  they  are  nourished.  They  are  born  in  a  rela- 
tively perfect  condition.  There  is  no  marsupial  pouch. 
The  following  orders  are  included  :  — 

i.    Edentata. — This  strange  order  contains  very  di- 
verse forms,  as  the  leaf-eating  sloths  and  the  insectivo- 
rous   ant  -  eaters     and 
armadillos     of     South 
America,  and  the  pan- 
golin and  orycteropus 

Of  the  Old  World.  The  FIG  168.  —  Skull  of  the  great  ant-eater  (Myrrne- 
o-io-antiV  fr»ccilc  rn^cro  cophaga  jubata} :  15,  nasal ;  n,  frontal;  7,  parietal; 
gig  dUlll  l  US,  i  ic^d-  3>  superoccipital;  2,  occipital  condyles;  28,  tym- 
therium  and  P"lvDtodon  panic;  73,  lachrymal;  32,  lower  mandible.  Teeth 
.  wanting. 

belong   to  this  group. 

The  sloths  and  ant-eaters  are  covered  with  coarse  hair ; 
the  armadillos  and  pangolins,  with  an  armor  of  plates 
or  scales  (Fig.  298).  The  ant-eaters  and  pangolins 
are  strictly  edentate,  or  toothless  ;  the  rest  have  molars, 
wanting,  however,  enamel  and  roots.  In  general,  it 


1 82      STRUCTURAL  AND   SYSTEMATIC  ZOOLOGY 

may  be  said  that  the  order  includes  all  quadrupeds 
having  separate,  clawed  toes  and  no  incisors.  The 
sloths  are  arboreal ;  the  others  burrow.  The  brain  is 
generally  smooth  ;  but  that  of  the  ant-eater  is  convoluted, 
and  has  a  large  corpus  callosum  ;  but  in  all  the  cerebel- 
lum and  part  of  the  olfactory  lobes  are  exposed. 


FIG.  169.  — Armadillo  (Dasypus).     Copyright,  1901,  by  N.Y.  Zoological  Society. 

2.  Cetacea,  or  whales,  have  the  form  and  life  of  fishes, 
yet  they  possess  a  higher  organization  than  the  preceding 
orders.  They  have  a  broad  brain,  with  many  and  deep 
foldings ;  the  foramen  magnum  of  the  skull  is  entirely 
posterior ;  the  whole  head  is  disproportionately  large,  and 
the  jaws  greatly  prolonged.  The  body  is  covered  with  a 
thick,  smooth  skin,  with  a  layer  of  fat  ("  blubber  ")  under- 
neath ;  there  are  no  clavicles  ;  the  hind  limbs  are  want- 
ing, and  the  front  pair  changed  to  paddles  (Fig.  171); 
the  tail  expands  into  a  powerful,  horizontal  fin  ;  neck 
and  external  ears  are  wanting  ;  the  eyes  small,  with  only 
two  lids  ;  the  nostrils  (blowholes)  —  double  in  the  whale, 


VERTEBRATA 


183 


single  in  the  porpoise  —  are  on  the  top  of  the  head.  All 
are  carnivorous,  and  essentially  marine,  a  few  dolphins 
only  being  found  in  the  great'rivers.  In  the  whalebone 


FIG.  170.  — Outline  of  the  sperm  whale  (Physeter).  The  blowhole  is  seen  at  the  extreme 
tip  of  the  head.  In  this  region  the  spermaceti  is  found.  Maximum  length,  eighty- 
five  feet.  South  Atlantic. 


whales,  the  teeth  are  absorbed,  and  disappear  before 
birth,  and  their  place  is  supplied  by  horny  "baleen" 
plates  (Fig.  228).  "  The  whale  feeds  by  putting  this 


FlG.  171.  —  Greenland  whale  (Baleena  mystzcetns} .     North  Atlantic. 

gigantic  strainer  into  operation,  as  it  swims  through  the 
shoals  of  minute  mollusks,  crustaceans,  and  fishes,  which 
are  constantly  found  at  the  surface  of  the  sea.  Open- 


1 84     STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

ing  its  capacious  mouth,  and  allowing  the  sea  water, 
with  its  multitudinous  tenants,  to  fill  the  oral  cavity,  the 
whale  shuts  the  lower  jaw  upon  the  baleen  plates,  and 
straining  out  the  water  through  them,  swallows  the  prey 
stranded  upon  its  vast  tongue."  In  the  other  cetaceans 
teeth  are  developed,  especially  in  dolphins  and  porpoises  ; 
but  the  sperm  whale  has  them  only  in  the  lower  jaw, 
and  the  narwhal  can  show  but  a  single  tusk.  In  the 
toothed  cetaceans  the  organ  of  smell  is  very  rudimentary 
or  even  absent  (dolphins). 

3.    Sirenia  resemble  the  cetaceans  in  shape,  but  are 
closely  allied  to   the   hoofed    animals   in    organization. 


FIG.  172.  —  Troop  of  dolphins,  with  manatee  in  the  distance. 

They  have  the  limbs  of  the  whales,  and  are  aquatic ;  but 
they  are  herbivorous,  and  frequent  great  rivers  and 
estuaries.  They  have  two  sets  of  teeth,  the  cetaceans 
having  but  one.  They  have  a  narrow  brain  ;  bristles 
scantily  covering  the  body ;  and  nostrils  placed  on  the 


VERTEBRATA  185 

snout,  which  is  large  and  fleshy.  The  living  representa- 
tives are  the  manatee,  of  both  sides  of  the  tropical  Atlantic 
Ocean,  and  the  dugong  of  the  East  Indies  (Fig.  270). 

4.  Ungulata,  or  hoofed  quadrupeds.  This  large 
order,  comprehending  many  animals  most  useful  to 
man,  is  distinguished  by  four  well-developed  limbs,  each 
toe  being  generally  encased  in  a  hoof  (Fig.  300).  The 
leg,  therefore,  has  no  prehensile  power ;  it  is  only  for 
support  and  locomotion.  Clavicles  are  wanting ;  and 
the  radius  and  ulna  are  so  united  as  to  prevent  rotation 


FIG.  173.  —  Indian  rhinoceros  (/?.  indicus). 

(Figs.  314,  316).  There  are  always  two  sets  of  teeth, 
i.e.,  milk  teeth  are  succeeded  by  a  permanent  set.  The 
grinders  have  broad  crowns  (Figs.  234,  308).  As  a  rule 
all  are  herbivorous.  The  brain  is  always  convoluted, 
but  the  cerebellum  is  largely  uncovered  (Fig.  335). 

Ungulates  are  divided  into  two  groups:  those  in 
which  the  feet  are  always  digitigrade,  with  never  more 
than  four  functional  digits,  as  the  horse,  ox,  and  rhinoce- 
ros; and  those  in  which  the  feet  may  be  plantigrade  with 
four  or  five  digits,  as  the  cony  (Hyrax\  of  Syria,  and 
the  elephant.  The  dental  formula  of  the  horse  is  — 


1  86     STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 

The  canines  are  often  wanting  in  the  mare.  The  horse49 
walks  on  the  third  finger  arid  toe.  The  metacarpals  and 
metatarsals  are  greatly  elongated,  so  that  the  wrist  and 
heel  are  raised  to  the  middle  of  the  leg  (Fig.  314). 
The  rhinoceros  and  tapir50  each  have  three  toes.  The 
first  is  distinguished  by  its  very  thick  skin,  the  absence 
of  canines,  and  one  or  two  horns  on  the  nose.  The 
tapir  has  the  four  kinds  of  teeth,  and  a  short  proboscis. 
The  Even-toed  Ungulates  —  hog,  hippopotamus,  and 
ruminants  —  have  two  or  four  toes.  The  hog  and  hip- 
popotamus have  the  four  kinds  of  teeth  (Fig.  232);  and, 
in  the  wild  state,  are  vegetarian.  The  ruminants  have 
two  toes  on  each  foot,  enveloped  in  hoofs  which  face 
each  other  by  a  flat  side,  so  that  they  appear  to  be  a 
single  hoof  split  or  "cloven."  Usually  there  are  also 
two  supplementary  hoofs  behind,  but  they  do  not  ordi- 
narily touch  the  ground.  All  chew  the  cud,  and  have  a 
complicated  stomach  (Fig.  254).  They  have  incisors  in 
the  lower  jaw  only,  and  these  are  apparently  eight;  but 
the  two  outer  ones  are  canines.51  The  molars  are  flat, 
typical  grinders.  The  dental  formula  of  the  ox  is  — 


o  —  o        o  —  o 

" 


3 

With  few  exceptions,  as  the  camel,  all  ruminants  have 
horns,  which  are  always  in  pairs.  Those  of  the  deer 
are  solid,  bony,  and  deciduous  ;  those  of  the  giraffe  and 
antelope  are  solid,  horny,  and  permanent;  in  the  goat, 
sheep,  and  ox,  they  are  hollow,  horny,  and  permanent. 

The  elephant,  now  nearly  extinct,  is  characterized  by 
two  upper  incisors  in  the  form  of  tusks,  mainly  com- 
posed of  dentine  (ivory).  In  the  extinct  dinotherium 
the  tusks  projected  from  the  lower  jaw  ;  and  in  the 
mastodon,  from  both  jaws.  .  Canines  are  wanting.  The 
molars  are  few  and  large,  with  transverse  ridges  (ele- 
phant) or  tubercles  (mastodon).  The  cerebrum  is  large 


VERTEBRATA  187 

and  convoluted,  but  does  not  cover  the  cerebellum.  The 
skull  is  enormous,  the  size  arising  in  great  measure  from 
the  development  of  air  cavities  between  the  inner  and 
outer  plates.  The  nose  is  prolonged  into  a  flexible 
trunk,  which  is  a  strong  and  delicate  organ  of  prehen- 
sion. There  are  four  massive  limbs,  each  with  five  toes 
incased  in  broad,  shallow  hoofs,  and  also  with  a  thick, 


FIG.  174.  —Stag,  or  red  deer  (Cervus  elaphus).    Europe.     Copyright,  1901,  by 
N.Y.  Zoological  Society. 

tegumentary  pad.  The  knee  is  below  and  free  from  the 
body,  as  in  monkeys  and  men.  Clavicles  are  wanting 
(Fig.  316).  The  body  of  the  elephant  is  nearly  naked ; 
but  the  mammoth,  an  extinct  species,  had  a  covering  of 
long  woolly  hair.  Elephants  live  in  large  herds,  and 
subsist  on  foliage  and  grass.  There  are  but  two  living 
species :  the  Asiatic,  with  long  head,  concave  forehead, 
small  ears,  and  short  tusks ;  and  the  African,  with  round 
head,  convex  forehead,  large  ears,  and  long  tusks.52 


1 88      STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


5.  Carnivora,  or  beasts  of  prey,  may  be  recognized  by 
their  four  long,  curved,  acute,  canine  teeth,  the  gap  be- 
tween the  incisors 
and  canines  in  the 
upper  jaw  for  the 
reception  of  the 
lower  canine,  and 
molars  graduating 
from  a  tuberculate 
to  a  trenchant  form, 
in  proportion  as 
the  diet  deviates 
from  a  miscella- 
neous kind  to  one 
strictly  of  flesh 
(Figs.  303,  307). 
The  incisors,  ex- 
cept in  the  pinni- 
grades,  number  six 
in  each  jaw.  There 
are  always  two 
sets.  The  skull  is 
comparatively 

FIG.  176.  —  Wolf  (Lupus  occidentalis}.     United  States. 
Copyright,  1901,  by  N.Y.  Zoological  Society. 


FIG.  175.  —  Raccoon  (Procyon  lotor).     United  States. 


the  jaWS  are 

shorter  and  deeper 
than  in  ungulates, 
and  there  are  nu- 
merous bony  ridges 
on  the  inside  and 
outside  of  the  cra- 
nium —  the  high 

FIG.  177.  —  Ermine  weasel  (Putorius  noveboracensis) .      QCCiDital    CTCSt     be- 
United  States. 

ing  specially  char- 
acteristic. The  cerebral  hemispheres  are  joined  by 
a  large  corpus  callosum,  but  the  cerebellum  is  never 


VERTEBRATA 


I89 


completely  covered  (Fig.  339).     Both  pairs  of  limbs  are 
well   developed,   the   front   being    prehensile ;   but  the 
clavicles  are  rudimentary.      The    humerus   and  femur 
are  mainly  inclosed 
in  the  body.     The 
digits,    never    less 
than   four,   always 
have     sharp     and 
pointed       claws.53 
The   body  is   cov- 
ered with  abundant 
hair. 

Carnivores  may 
be  divided  accord- 
ing to  the  modifica-  FIG.  178.  —  Kz&foxtVulpespennsylvanicus).  United 

tions  of  the  limbs  :     States"  Co^^ht'  ^OI> b^  N- Y-  Zo°logical  Society 
a.  Pinnigrades,  having  short  feet  expanded  into  webbed 
paddles  for  swimming,  the  hinder  ones  being  bound  in 
with  the  skin  of  the  tail.    -Such  are  the  seals,  walrus, 


FIG.  179.  — Southern  sea  lion  (Otan'a  jubata) .     Antarctic  Ocean. 

and  eared  seals,  or  sea  lions,  b.  Plantigrades,  in  which 
the  whole,  or  nearly  the  whole,  of  the  hind  foot  forms 
a  sole,  and  rests  on  the  ground.  The  claws  are  not 


190     STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 

retractile ;  the  ears  are  small,  and  tail  short.  Bears,  bad- 
gers, and  raccoons  are  well-known  examples,  c.  Digiti- 
grades  keep  the  heel  raised  above  the  ground,  walking 
on  the  toes.  The  majority  have  long  tails.  Such  are 
the  weasels,  otters,  civets,  hyenas,  foxes,  jackals,  wolves, 
dogs,  cats,  panthers,  leopards,  tigers,  and  lions.  .  The 
last  five  differ  from  all  others  in  having  retractile 
claws,  and  the  radius  rotating  freely  on  the  ulna.  The 
cats  have  thirty  teeth ;  the  dogs,  forty-two,  or  twelve 
more  molars.  In  the  former,  the  tongue  is  prickly ;  in 
the  latter,  smooth. 

6.  Rodentia,  or   gnawers,  are    characterized   by  two 
long,  curved  incisors  in  each   jaw,  enameled  in   front, 


FIG.  180.  — Skull  of  a  rodent  (Capybara)  :  22,  premaxillary;  21,  maxillary;  26,  molar; 
27,  squamosal;  73,  lachrymal;  15,  nasal;  n,  frontal;  4,  occipital  processes,  un- 
usually developed;  z,  incisors;  a,  angle  of  lower  jaw. 

and  perpetually  growing ;  they  are  specially  formed  for 
nibbling.  Separated  from  them  by  a  wide  space  (for 
canines  are  wanting),  are  the  flat  molars,  admirably 
fitted  for  grinding.  The  lower  jaw  has  longitudinal 
condyles,  which  work  freely  backward  and  forward  in 
longitudinal  furrows.  Nearly  all  have  clavicles,  and 
the  toes  are  clawed.  The  cerebrum  is  nearly  or  quite 
smooth,  and  covers  but  a  small  part  of  the  cerebellum. 
All  are  vegetarian. 

More  than  one  half  of  all  known  mammals  are  rodents. 


VERTEBRATA 


They  range  from  the  equator  to  the  poles,  over  every 
continent,  over  mountains  and  plains,  deserts  and  woods. 


FIG.  181.  —  Incisor  teeth  of  the  hare. 


The  more  important  representatives  are  the  porcupines, 
capybaras,  guinea-pigs,  hares,  mice,  rats,  squirrels,  and 


FIG.   182.  —  Beaver   {Castor  canadensis).     North   America.     Copyright,   1900,  by 
A.  Radcliffe  Dugmore. 

beavers.     The  capybara  and  beaver  are  the  giants  of 
the  race. 


192      STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


7.  Insectivora    are  diminutive,  insect-eating   animals, 
some,  as   the  shrew,  being   the  smallest  of  mammals. 

They  have  small,  smooth  brains, 
which,  as  in  the  preceding  or- 
ders, leave  uncovered  the  cere- 
bellum and  olfactory  lobes.  The 
molar  teeth  bristle  with  sharp, 
pointed  cusps,  and  are  asso- 
FIG.  ,83.-  shrew  mouse  (Sor**).  c^^  with  canines  and  incisors. 

They  have  a  long  muzzle,  short  legs,  and  clavicles. 
The  feet  are  formed  for  walking  or  grasping,  and  are 
plantigrade,  five-toed,  and  clawed.  The  shrew,  hedge- 
hog, and  mole  are  examples. 

8.  Cheiroptera,  or  bats,  repeat  the  chief  characters  of 
the  Insectivores ;  but  some  (as  the  flying  fox)  are  fruit- 
eaters,    and   have   corresponding   modifications   of   the 


FIG.  184.  —  Bat  (.Vespertilid). 

teeth.  They  are  distinguished  by  their  very  long  fore 
limbs,  which  are  adapted  for  flight,  the  fingers  being  im- 
mensely lengthened,  and  united  by  a  membranous  web. 


VERTEBRATA 


193 


The  toes,  and  one  or  two  of  the  fingers,  are  armed  with 
hooked  nails.  The  clavicles  are  remarkably  long,  and 
the  sternum  is  of  great  strength ;  but  the  whole  skele- 
ton is  extremely  light,  though  not  filled  with  air,  as  in 


\  ,. 


FIG.  185.  —  Skeleton  of  a  bat. 

birds.  The  eyes  are  small,  the  ears  large,  and  the  sense 
of  touch  is  very  acute.  The  favorite  attitude  of  a  bat 
when  at  rest  is  that  of  suspension  by  the  claws,  with 
head  downward.  They  are  all  nocturnal. 

9.  Primates,  the  head  of  the  kingdom,  are  character- 
ized by  the  possession  of  two  hands  and  two  feet.  The 
thigh  is  free  from  the  body,  and  all  the  digits  are  fur- 
nished with  nails,  the  first  on  the  foot  enlarged  to  a 
"great  toe."  Throughout  the  order,  the  hand  is  emi- 
nently or  wholly  prehensile,  and  the  foot,  however 
prehensile  it  may  be,  is  always  a  locomotor  organ  (Fig. 
IQ2).54  The  clavicles  are  perfect  (Fig.  317).  The  eyes 
are  situated  in  a  complete  bony  cavity,  and  look  for- 
ward. There  are  two  sets  of  teeth,  all  enameled ;  and 
the  incisors  number  four  in  each  jaw  (Fig.  233).  They 
include  the  lemurs,  monkeys  and  apes,  and  man. 

The  lemurs  are  covered  with  soft  fur,  have  usually 

a  long  tail,    pointed   ears,  foxlike  muzzle,   and  curved 

nostrils.      They  walk  on  all  fours,  and  the  thumb  and 

great  toe  are    generally  opposable  to  the  digits:     The 

DODGE'S  GEN.  ZOOL.  —  13 


194     STRUCTURAL   AND   SYSTEMATIC   ZOOLOGY 


second  toe  has  a  long,  pointed  claw  instead  of  a  nail. 

The    cerebrum   is   relatively  small,  and   flattened,  and 

does  not  cover  the 
cerebellum  and  olfac- 
tory lobes.55  They 
are  found  mainly  in 
Madagascar. 

The  monkeys  of 
tropical  America 
have,  generally,  a 
long,  prehensile  tail; 
the  nostrils  are  placed 

FIG.    186.  —  Lemur    (L.     ruber).      Madagascar.       far  apart,    SO  that    the 
Copyright,  1901,  by  N.  Y.  Zoological  Society. 

nose  is  wide  and  flat ; 
the  thumbs  and  great  «toes  are  fitted  for  grasping,  but  are 


1   FIG.  187.  —  White-throated  sapajou  (Cebus  hypoleucus).     Central  America. 

not  opposable  to  the  other  digits ;    and  they  have  four 
molars  more  than  the  apes  or  man  —  that  is,  thirty-six 


VERTEBRATA 


195 


teeth  in  all.    In  the  apes  of  the  Old  World  the  tail  is  never 
prehensile,  and  is  sometimes  wanting;  the  nostrils  are 


FIG.  188.  —  Skull  of  orang-outang  (Simia        FIG.  189.  —  Skull  of  chimpanzee  (Anthro~ 
satyrus}.  popithecus  troglodytes). 

close  together ;  both  thumbs  and  great  toes  are  opposa- 
ble ;  and  the  teeth,  though  numbering  the  same  as  man's, 


FIG.  190. — Female  orang-outang  (from  photograph).     Borneo. 

are  uneven  (the  incisors  being  prominent,  and  the  canines 
large),  and  the  series  is  interrupted  by  a  gap  on  one  side 


196     STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 

or  other  of  the  canines.  Their  average  size  is  much 
greater  than  that  of  the  monkeys,  and  they  are  not 
so  strictly  arboreal.  In  both  monkeys  and  apes  the 
cerebrum  covers  the  cerebellum  (Fig.  34O).56  While 
in  the  monkeys  the  skull  is  rounded  and  smooth,  that 
of  the  apes,  especially  those  coming  nearest  to  man  — 
the  anthropoid,  or  long-armed,  apes,  as  gorilla,  chim- 
panzee, orang,  and  gibbon  —  is  characterized  by  strong 
crests.  Monkeys  take  a  horizontal  position ;  but  the 
apes  assume  a  semierect  attitude,  the  legs  being  shorter 
than  the  arms.  In  all  the  primates  but  man,  the  body 
is  clothed  with  hair,  which  is  generally  longest  on  the 
back.  Several  monkeys  and  apes  have  a  beard,  as  the 
howler  and  orang. 

The  orang  is  the  least  human  of  all  the  anthropoid 


FIG.  191.  —  Skeletons  of  man,  chimpanzee,  and  orang. 

apes  as  regards  the  skeleton,  but  comes  nearest  to  man 
in  the  form  of  the  brain.  The  chimpanzee  approaches 
man  more  closely  in  the  character  of  its  cranium  and 
teeth,  and  the  proportional  length  of  the  arms.  The 


VERTEBRATA  197 

gorilla  is  most  manlike  in  bulk  (sometimes  reaching 
the  height  of  five  feet  six  inches),  in  the  proportions 
of  the  leg  to  the  body  and  of  the  foot  to  the  hand,  in 
the  size  of  the  heel,  the  form  of  the  pelvis  and  shoulder 
blade,  and  volume  of  brain.67 


FIG.  192  —  Gorilla. 

Man  differs  from  the  apes  in  being  an  erect  biped. 
In  him,  the  vertebrate  type,  which  began  in  the  hori- 
zontal fish,  finally  became  vertical.  No  other  animal 
habitually  stands  erect ;  in  no  other  are  the  fore  limbs 
used  exclusively  for  prehensile  purposes,  and  the  hind 
pair  solely  for  locomotion. 

His  limbs  are  naturally  parallel  to  the  axis  of  his 
body,  not  perpendicular.  They  have  a  near  equality 
of  length,  but  the  arms  are  always  somewhat  shorter 
than  the  legs.  In  all  the  great  apes  the  arms  reach 
below  the  knee,  and  the  legs  of  the  chimpanzee  and 
gorilla  are  relatively  shorter  than  man's. 

Only  man  has  a  finished   hand,  most  perfect  as  an 


198     STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 

organ  of   touch,  and  most  versatile.     Both    hand   and 
foot  are  relatively  shorter  than  in  the  apes.     The  foot 


a  b 

FIG.  193.  —  Foot  (a)  and  hand  (b)  of  the  gorilla. 

is  plantigrade ;  the  leg  bears  vertically  upon  it ;  the 
heel  and  great  toe  are  longer  than  in  other  primates ; 
and  the  great  toe  is  not  opposable,  but  is  used  only 
as  a  fulcrum  in  locomotion.  The  gorilla  has  both  an 


FIG.  194.  —  Australian  savage. 

inferior  hand  and  inferior  foot.     The  hand  is  clumsier, 
and    with    a    shorter    thumb   than    man's;     and    the 


VERTEBRATA 


I99 


foot  is  prehensile,  and  is  not  applied  flat  to  the 
ground.58 

The  scapular  and  pelvic  bones  are  extremely  broad, 
and  the  neck  of  the  femur  remarkably  long.  Man  is 
also  singular  in  the  double  curve  of  the  spine :  the 
baboon  comes  nearest  to  man  in  this  respect. 

The  human  skull  has  a  smooth,  rounded  outline,  ele- 
vated in  front,  and  devoid  of  crests.  The  cranium 
greatly  predominates  over  the  face,  being  four  to  one ; 59 
and  no  other  animal  (except  the  siamang  gibbon)  has  a 
chin. 

Man  stands  alone  in  the  peculiarity  of  his  dentition : 


FIG.  195.  —  Skull  of  European. 


FIG.  196.  —  Skull  of  negro. 


his  teeth  are  vertical,  of  nearly  uniform  height,  and  close 
together.  In  every  other  animal  the  incisors  and  canines 
are  more  or  less  inclined,  the  canines  project,  and  there 
are  vacant  spaces.60 

Man  has  a  longer  lobule  to  his  ear  than  any  ape,  and 
no  muzzle.  The  bridge  of  his  nose  is  decidedly  convex ; 
in  the  apes  generally  it  is  flat. 

Man  has  been  called  the  only  naked  terrestrial  mam- 
mal. His  hair  is  most  abundant  on  the  scalp  ;  never  on 
the  back,  as  in  the  apes. 

Man  has  a  more  pliable  constitution  than  the  apes,  as 


200     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 

shown  by  his  world-wide  distribution  The  animals  near- 
est him  soon  perish  when  removed  from  their  native 
places. 

Though  man  is  excelled  by  some  animals  in  the  acute- 
ness  of  some  senses,  there  is  no  other  animal  in  which 
all  the  senses  are  capable  of  equal  development.  He 
alone  has  the  power  of  expressing  his  thoughts  by 
articulate  speech,  and  the  power  of  forming  abstract 
ideas. 

Man  differs  from  the  apes  in  the  absolute  size  of  the 
brain,  and  in  the  greater  complexity  and  less  symmet- 
rical disposition  of  its  convolutions.  The  cerebrum  is 
larger  in  proportion  to  the  cerebellum  (being  as  8|-  to  i), 
and  the  former  not  only  covers  the  latter,  but  projects 
beyond  it.  The  brain  of  the  gorilla  scarcely  amounts 
to  one  third  in  volume  or  one  half  in  weight  of  that  of 
man.  Yet,  so  far  as  cerebral  structure  goes,  man  differs 
less  from  the  apes  than  they  do  from  the  monkeys  and 
lemurs. 

The  view  held  by  evolutionists  that  man  and  the  man- 
like apes  are  descendants  of  a  common  ancestor  is  based 
upon  arguments  drawn  from  structural  and  physiologi- 
cal features.  In  his  anatomy  man  resembles  apes  more 
closely  than  any  other  group  of  animals.  He  differs 
from  them  mainly  in  having  a  much  larger  brain.  In 
his  skeletal,  muscular,  nervous,  and  other  systems  he 
possesses  about  seventy-five  vestigial  structures,  i.e., 
anatomical  parts  which  are  more  perfectly  developed 
and  more  useful  in  apes  and  lower  animals.  Physiologi- 
cally, man  resembles  the  apes  in  having  a  similar  bodily 
life,  in  performing  many  actions  in  the  same  manner, 
in  being  subject  to  the  same  diseases,  in  making 
similar  gestures,  facial  expressions,  etc.  The  great 
gulf  between  man  and  the  brute  is  not  physical,  but 
psychical,61 


THE    CLASSIFICATION    OF   ANIMALS 


2O I 


Meso:zo^>vJ'' 

^fcdS^^Hiw 

PfroVoZ  ^ 


4       ^- 

j.  V 

X^|V  ^<X 


FIG.  197.  — Diagrammatic  expression  of  classification  in  a  genealogical  tree.     B  indicates 
possible  position  of  Balanoglossus,  D  of  Dipnoi,  S  of  Sphenodon  or  Hatteria. 


202     STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


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:  Gregarina, 

tentacles. 

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ARRANGEMENT   OF   REPRESENTATIVE   FORMS     203 


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204     STRUCTURAL  AND    SYSTEMATIC   ZOOLOGY 


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ARRANGEMENT   OF    REPRESENTATIVE   FORMS     205 


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206     STRUCTURAL  AND   SYSTEMATIC   ZOOLOGY 


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:t  ;  chitinous  crust  ;  six  thoracic  legs  ;  winged  ;  two  antennae; 

lickened,  narrow  and  overlapping,  hind  pair  transparent,  broad,  and  folded; 
•ge,  transparent  wings;  biters:  Libellula, 

is  :  Cimex. 
(  transparent:  Cicada. 
r  forewings  opaque  at  base  :  A  nasa. 
;  suctorial:  Musca. 
f  antennae  feathery  :  Telea. 

.  ,  .  r  antennae  spindle-shaped: 
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\  fitted  for  both  biting  and  suction:  Apis. 

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nnerets;  pulmonary  sacs  :  Epeira. 
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208      STRUCTURAL   AND    SYSTEMATIC   ZOOLOGY 


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lartilaginous  skeleton;  tail  heterocercal  ;  gills  fixed  and  uncovered:  Sgualus. 

(  soft-finned  :  Salmo. 
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210     STRUCTURAL  AND   SYSTEMATIC  ZOOLOGY 


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ARRANGEMENT   OF   REPRESENTATIVE   FORMS     211 


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ATA.  —  Duck-billed;  webbed  feet:  Ornithorhy 
SUBCLASS  \\.-Theria. 

Pouched  Mammalia, 
IA.  —  With  pouch  for  immature  young  :  Didel^ 

/>7x,x--,«/^7  M~*»,  «x,7»/, 

(  Toothless:  Myrmecophaga. 
'  Incisors  wanting:  Bradvfius. 

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nostrils  on  top  of  the  head;  carnivorous;  J 

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—  Canines  wanting:  incisors  highly  developed: 
A.  —Molars  with  sharp  points:  Scalofis. 

RA.  —  Fore  limbs  webbed  for  flight:  Vespertih 

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PART    II 
COMPARATIVE   ZOOLOGY 


CHAPTER    IV 

MINERALS   AND   ORGANIZED   BODIES   DISTINGUISHED 

Nature  may  be  separated  into  two  great  kingdoms, — 
that  of  mere  dead  matter,  and  that  of  matter  under  the 
influence  of  life.62  These  differ  in  the  following  points : — 

(i)  Composition. — While  most  of  the  chemical  elements 
are  found  in  different  living  beings,  by  far*  the  greater 
part  of  their  substance  is  composed  of  three  or  four,  — 
carbon,  oxygen,  and  hydrogen ;  or  these  three  with  the 
addition  of  nitrogen.  Next  to  these  elements,  sulphur 
and  phosphorus  are  most  widely  distributed,  though 
always  found  in  very  small  quantities.  The  organic  com- 
pounds belong  to  the  carbon  series,  and  contain  three, 
four,  or  five  elements.  The  former  class,  comprising 
starch,  sugar,  fat,  etc.,  are  relatively  stable.  The  latter, 
possessing  the  three  elements  named,  with  nitrogen  and 
sulphur  or  phosphorus,  are  very  complex,  containing  a 
very  large  number  of  atoms  to  the  molecule,  and  are 
usually  unstable.  Here  belong  albumen,  myosin,  chon- 
drin,  etc.,  the  constituents  of  the  living  tissues.  The 
formula  for  albumen  is  said  to  be  C72H112N18SO22,  or 
some  multiple  of  this  formula.  These  compounds  also 
contain  more  or  less  water,  and  usually  exist  in  a  jelly- 
like  condition,  neither  solid  nor  fluid.  All  organic  com- 
pounds are  formed  through  the  chemical  activities  of 
protoplasm,  which  is  the  only  living  substance.  Inor- 
ganic matter  may,  under  its  influence,  be  changed  to 
organic,  and  vice  versa ;  dead  matter  which  enters  the 
body  of  organized  beings  in  the  form  of  nutriment  is 

215 


2l6  COMPARATIVE   ZOOLOGY 

changed  into  living  substance,  which,  after  serving  its 
purpose,  passes  again  as  waste  to  the  inorganic  world. 

(2)  Structure. —  Minerals  are  homogeneous,  while  organ- 
ized bodies  are  usually  heterogeneous,  i.e.,  composed  of 
different  parts,  called  tissues  and  organs,  having  peculiar 
uses  and  definite  relations  to  one  another.     The  tissues 
and  organs,  again,  are  heterogeneous,  consisting  mainly 
of  microscopic  cells,  structures  developed  only  by  vital 
action.     All  the   parts   of   an   organism   are   mutually 
dependent,   and    reciprocally   means    and   ends,    while 
each  part  of  a  mineral  exists  for  itself.     The  smallest 
fragment  of  marble  is  as  much  marble  as  a  mountain 
mass ;  but  the  fragment  of  a  plant  or  animal  is  not  an 
individual. 

(3)  Shape  and  Size.  —  Living  bodies  gradually  acquire 
determinate   dimensions ;    so  do  minerals  in  their  per- 
fect or  crystal  condition.     But  uncrystallized,  inorganic 
bodies   have    an    indefinite   bulk.      Most   minerals   are 
amorphous ;  crystals  have  regular  forms,  bounded,  as  a 
rule,  by  plane  surfaces  and  straight  lines ;  plants  and 
animals    are    circumscribed    by    curved    surfaces,    and 
rarely  assume  accurate  geometrical  forms.  63 

(4)  Phenomena.  —  Minerals  remain  internally  at  rest,  and 
increase  by  external  additions,  if  they  grow  at  all.     Liv- 
ing beings  are  constantly  changing  the  matter  of  which 
they  are  composed,  and  grow  by  taking  new  matter  into 
themselves  and  placing  it  among  the  particles  already 
present.     Organized  bodies,  moreover,  pass  through  a 
cycle  of  changes,  —  growth,  development,  reproduction, 
and  death.     These  phenomena  are  characteristic  of  liv- 
ing as  opposed  to  inorganic  bodies.     All  living  bodies 
grow  from  within,  constantly  give   up  old  matter  and 
replace  it  by  new,  reproduce  their  kind,  and  die  ;  and 
no  inorganic  body  shows  any  of  these  phenomena. 


CHAPTER   V* 

PLANTS   AND   ANIMALS   DISTINGUISHED 

IT  may  seem  an  easy  matter  to  draw  a  line  between 
plants  and  animals.  Who  cannot  tell  a  cow  from  a 
cabbage  ?  Who  would  confound  a  coral  with  a  mush- 
room ?  Yet  it  is  impossible  to  assign  any  absolute,  dis- 
tinctive character  which  will  divide  the  one  form  of  life 
from  the  other.  The  difficulty  of  defining  an  animal 
increases  with  our  knowledge  of  its  nature.  Linnaeus 
denned  it  in  three  words  ;f  a  century  later,  Owen  declared 
that  a  definition  of  plants  which  would  exclude  all 
animals,  or  of  animals  which  would  not  let  in  a  single 
plant,  was  impossible.  Each  different  character  used 
in  drawing  the  boundary  will  bisect  the  debatable  ground 
in  a  different  latitude  of  the  organic  world.  Between 
the  higher  animals  and  higher  plants  the  difference  is 
apparent ;  but  when  we  reflect  how  many  characters  the 
two  have  in  common,  and  especially  when  we  descend 
to  the  lower  and  minuter  forms,  we  discover  that  the 
two  "  kingdoms "  touch,  and  even  dissolve  into,  each 
other.  This  border  land  has  been  as  hotly  contested 
among  naturalists  as  many  a  disputed  frontier  between 
adjacent  nations.  Its  inhabitants  have  been  taken  and 
retaken  several  times  by  botanists  and  zoologists  ;  for 
they  have  characters  that  lead  on  the  one  side  to 
plants,  and  on  the  other  to  animals.  To  solve  the 
difficulty,  some  eminent  naturalists,  as  Haeckel  and 

*  See  Appendix. 

t  "  Minerals  grow  ;  plants  grow  and  live  ;  animals  grow,  live,  and 
feel." 

217 


2l8  COMPARATIVE   ZOOLOGY 

Owen,  propose  a  fourth  "  kingdom,"  that  of  the 
Protista,  to  receive  those  living  beings  which  are 
organic,  but  not  distinctly  vegetable  or  animal.  But  a 
greater  difficulty  arises  in  attempting  to  fix  its  precise 
limits. 

The  drift  of  modern  research  points  to  this :  that 
there  are  but  two  kingdoms  of  nature,  the  mineral  and 
the  organized,  and  these  closely  linked  together ;  that 
the  latter  must  be  taken  as  one  whole,  from  which  two 
great  branches  rise  and  diverge.  "  There  is  at  bottom 
but  one  life,  which  is  the  whole  life  of  some  creatures 
and  the  common  basis  of  the  life  of  all ;  a  life  of  sim- 
plest moving  and  feeling,  of  feeding  and  breathing,  of 
producing  its  kind  and  lasting  its  day :  a  life  which,  so 
far  as  we  at  present  know,  has  no  need  of  such  parts 
as  we  call  organs.  Upon  this  general  foundation  are 
built  up  the  manifold  special  characters  of  animal  and 
vegetable  existence  ;  but  the  tendency,  the  endeavor,  so 
to  speak,  of  the  plant  is  one,  of  the  animal  is  another, 
and  the  unlikeness  between  them  widens  the  higher  the 
building  is  carried  up.  As  we  pass  along  the  series  of 
either  [branch]  from  low  to  high,  the  plant  becomes 
more  vegetative,  the  animal  more  animal."64 

Defining  animals  and  plants  by  their  prominent  char- 
acteristics, we  may  say  that  a  living  being  which  has 
cell  walls  of  cellulose,  and  by  deoxidation  and  synthesis 
of  its  simple  food  stuffs  produces  the  complicated  or- 
ganic substances,  is  a  plant ;  while  a  living  being  which 
has  albuminous  tissues,  and  by  oxidation  and  analysis 
reduces  its  complicated  food  stuffs  to  a  simpler  form, 
is  an  animal.  But  both  definitions  are  defective,  includ- 
ing too  many  forms,  and  excluding  forms  that  properly 
belong  to  the  respective  kingdoms.  No  definition  is 
possible  which  shall  include  all  animals  and  exclude  all 
plants,  or  vice  versa. 


PLANTS   AND    ANIMALS  DISTINGUISHED        219 

(1)  Origin.  —  Both  branches  of   the  tree  of  life  start 
alike :  the    lowest   of   plants  and   animals  consist  of  a 
single  cell.     In  fact,  the  cycle  of  life  in  all  living  beings 
begins  in  a  small,  round  particle  of  matter,  a  cell  —  in 
the  higher  plants  called  an  ovule,  in  the  higher  animals 
an  ovum.     This  cell  consists  mainly  of  a  semifluid  sub- 
stance called  protoplasm.     In  the  very  simplest  forms 
the  protoplasm  is  not  inclosed  by  a  membrane  or  cell 
wall.     In  most  plants  the  cell  wall  is  present,  and  con- 
sists of  cellulose,  a  substance  akin  to  starch;  in  animals, 
with  few  exceptions,  the  wall  is  a  pellicle  of  firmer  pro- 
toplasm, i.e.,  albuminous. 

(2)  Composition.  —  Modern  research  has  broken  down 
the    partition   between   plants   and   animals,  so   far  as 
chemical   nature   is  concerned.     The   vegetable   fabric 
and  secretions  may  be  ternary  or  binary   compounds ; 
but  the  essential  living  parts  of  plants,  as  of  animals, 
are  quaternary,  consisting  of  four  elements,  —  carbon, 
hydrogen,    oxygen,    and    nitrogen.       Cellulose    (woody 
fiber),  starch,  and  chlorophyl  (green   coloring   matter) 
are  eminently  vegetable  products,  but  not  distinctive ; 
for  cellulose  is  wanting  in  some  plants,  as  some  fungi, 
and  present  in  some  animals,  as  tunicates  ;  starch,  under 
the  name  of  glycogen,  is  found  in  the  liver  and  brains 
of  mammals,  and  chlorophyl  gives  color  to  the  fresh- 
water polyp.     Still,  it  holds  good,  generally,  that  plants 
consist  mainly  of  cellulose,  dextrin,  and  starch  ;  while 
animals  are  mainly  made  up    of   albumen,  fibrin,    and 
gelatin ;  that  nitrogen  is  more  abundant  in  animal  tis- 
sues, while  in  plants  carbon  is  predominant. 

(3)  Form.  —  No  outline  can  be  drawn  which  shall  be 
common  to  all  animals  or  all  plants.     The  lowest  mem- 
bers of  each  group  have  no  fixed  shape.     The  spores 
of  Confervae  can  hardly  be  distinguished  from  animal- 
cules ;  the  compound  and  fixed   animals,  sea  mat  and 


220  COMPARATIVE   ZOOLOGY 

sea  moss  (Polyzoa),  and  corals,  often  resemble  vegetable 
forms,  although  in  structure  widely  removed  from 
plants.  Similar  conditions  of  life  are  here  accompanied 
by  an  external  likeness.  In  free-living  animals  this 
resemblance  is  not  found. 

(4)  Structure.  —  A  plant  is  the  multiplication  of  the 
unit  —  a  cell  with  a  cellulose  wall.     Some  simple  ani- 
mals have  a  similar  simple  cellular  structure  ;  and  all 
animal   tissues,  while  forming,  are   cellular.     But   this 
character,  which  is    permanent  in  plants,  is    generally 
transitory  in  animals.     In   the  more   highly  organized 
tissues  the  cells  are  so  united  as  partly  or  wholly  to  lose 
their  individuality,  and   the  characteristic  part   of   the 
tissue   is    the   intercellular   substance,    while   the   cells 
themselves  are  small  and  unimportant,  or  else  the  cells 
are  fused  together  and  their  dividing  walls  become  in- 
distinct, as  in  glandular  tissue.     Excepting  the  lowest 
forms,  animals  are   more   composite   than    plants,  i.e., 
their   organs   are   more   complex   and    numerous,    and 
more  specially  devoted  to  particular  purposes.     Repe- 
tition of  similar  parts  is  a  characteristic  of  plants ;  and 
when  found  in  animals,  as  the  angleworm,  is  called  vege- 
tative repetition.     Differentiation  and  specialization  are 
characteristic   of    animals.       Most   animals,    moreover, 
have  fore-and-aft  polarity  ;  in  contrast,    plants  are  up- 
and-down  structures,  though   in  this  respect   they  are 
imitated   by  radiate  animals,  like   the  starfish.     Plants 
are   continually   receiving   additional    members ;    most 
animals  soon  become  perfect. 

(5)  Physiology.  —  In  their  modes  of  nutrition,   plants 
and  animals  stand  widest  apart.     A  plant  in  the  seed 
and  an  animal  in  the  egg  exist  in  similar  conditions :  in 
both  cases  a  mass  of  organic  matter  accompanies  the 
germ.      When  this  supply  of  food  is  exhausted,  both 
seek   nourishment   from   without.      But    here   analogy 


PLANTS   AND   ANIMALS   DISTINGUISHED        221 

ends:  the  green  plant  feeds  on  mineral  matter,  the 
animal  on  organic.  Some  plants  have  the  power  to 
form  chlorophyl,  the  green  coloring  matter  of  leaves, 
which  uses  the  energy  of  the  sunlight  to  form  starch 
out  of  the  inorganic  substances,  —  carbon  dioxide  and 
water.  They  are  able  also  to  form  albuminoid  matter 
out  of  inorganic  substances.  A  very  few  animals  which 
have  a  substance  identical  with  or  allied  to  chlorophyl 
have  the  same  power,  but  in  general  animals  are  de- 
pendent for  their  food  on  the  compounds  put  together  in 
plants.  Colorless  plants,  as  fungi,  possessing  no  chloro- 
phyl, feed,  like  animals,  on  organic  compounds.  No 
living  being  is  able  to  combine  the  simple  elements  — 
carbon,  oxygen,  hydrogen,  and  nitrogen  —  into  organic 
compounds. 

The  food  of  plants  is  gaseous  (carbon  dioxide  and 
ammonia)  or  liquid  (water  containing  substances  in  so- 
lution), that  of  animals  usually  more  or  less  solid,  though 
solid  substances  must  be  changed  to  liquids  before  being 
capable  of  absorption  into  the  tissues.  The  plant,  then, 
absorbs  these  foods  through  its  outer  surface,  while  the 
animal  takes  its  nourishment  in  larger  or  smaller  masses, 
and  digests  it  in  a  special  cavity.  A  few  exceptions, 
however,  occur,  since  certain  animals,  as  the  tapeworm, 
have  no  digestive  tract  but  absorb  liquid  food  through 
the  surface  of  the  body. 

Plants  are  ordinarily  fixed,  their  food  is  brought  to 
them,  and  a  large  share  of  their  work,  the  formation  of 
organic  compounds,  is  done  by  the  energy  of  the  sun- 
light; while  animals  are  usually  locomotive,  must  seek 
their  food,  and  are  unable  to  utilize  the  general  forces 
of  nature  as  the  plant  does.  The  plant  is  thus  able  to 
grow  much  more  than  the  animal,  as  very  little  of  the 
nourishment  received  is  used  to  repair  waste,  while  in 
most  animals  the  time  soon  comes  when  waste  and  re- 


222  COMPARATIVE   ZOOLOGY 

pair  are  approximately  equal.  But  in  both  all  work 
done  is  paid  for  by  waste  of  substance  already  formed. 

In  combining  carbon  dioxide  and  water  to  form  starch 
the  plant  sets  oxygen  free  (6(CO2)  +  5(H2O)  =  C6H10O5 
+  6(O2)) :  in  oxidizing  starch  or  other  food  the  animal 
uses  oxygen  and  sets  carbon  dioxide  free.  The  green 
plant  in  the  sunlight,  then,  gives  off  oxygen  and  uses 
carbon  dioxide,  while  plants,  which  have  no  chlorophyl, 
at  all  times,  and  all  plants  in  the  darkness,  use  oxygen 
and  give  off  carbon  dioxide,  like  an  animal.  Every 
plant  begins  life  like  an  animal  —  a  consumer,  not  a 
producer :  not  till  the  young  shoot  rises  above  the  soil, 
and  unfolds  itself  to  the  light  of  the  sun,  at  the  touch 
of  whose  mystic  rays  chlorophyl  is  developed,  does  real, 
constructive  vegetation  begin ;  then  its  mode  of  life  is, 
in  a  sense,  reversed;  since  more  carbon  is  combined 
than  liberated,  and  more  oxygen  set  free  than  main- 
tained. 

Most  plants,  and  many  animals,  multiply  by  budding 
and  division  ;  on  both  we  practice  grafting ;  in  both  the 
cycle  of  life  comes  round  again  to  the  ovule  or  ovum. 
Do  annuals  flower  but  to  die  ?  Insects  lay  their  eggs 
in  their  old  age. 

Both  animals  and  plants  have  sensibility.  This  is  one 
of  the  fundamental  physiological  properties  of  proto- 
plasm. But  in  plants  the  protoplasm  is  scattered  and 
buried  in  rigid  structures :  feeling  is,  therefore,  dull. 
In  animals  irritability  is  a  highly  developed  property  of 
certain  organs,  and  so  feeling,  like  electricity  rammed 
into  Ley  den  jars,  goes  off  with  a  flash.65  Plants  prob- 
ably never  possess  consciousness  or  volition,  as  the 
higher  animals  do. 

The  self-motion  of  animals  and  the  rooted  state  of 
plants  is  a  very  general  distinction  ;  but  it  fails  where 
we  need  it  most.  It  is  a  characteristic  of  living  things 


PLANTS   AND    ANIMALS   DISTINGUISHED        223 

to  move.  The  protoplasm  of  all  organisms  is  unceas- 
ingly active.66  Besides  this  internal  movement,  myriads 
of  plants,  as  well  as  animals,  are  locomotive.  Rambling 
diatoms,  writhing  oscillaria,  and  the  agile  spores  of  cryp- 
togams crowd  our  waters,  their  organs  of  motion  (cilia 
and  pseudopodia)  being  of  the  very  same  character  as 
in  microscopic  animals ;  while  sponges,  corals,  oysters, 
and  barnacles  are  stationary.  A  contractile  vesicle  is 
not  exclusively  an  animal  property,  for  the  several  fresh- 
water algae,  as  Gonium,  have  it.  The  muscular  contrac- 
tions of  the  highest  animals  and  the  sensible  motions  of 
plants  are  both  due  to  changes  in  the  protoplasm  in 
their  cells.  The  ciliary  movements  of  animals  and  of 
microscopic  plants  are  precisely  similar,  and  in  neither 
case  necessarily  indicate  consciousness  or  self-determin- 
ing power. 

Plants,  as  well  as  animals,  need  a  season  of  repose. 
Both  have  their  epidemics.  On  both,  narcotic  and 
acrid  poisons  produce  analogous  results.  Are  some 
animals  warm-blooded  ?  In  germination  and  flowering, 
plants  evolve  heat  —  the  stamens  of  the  arum,  e.g., 
showing  a  rise  of  20°  F.  In  a  sense,  an  oak  has  just 
as  much  heat  as  an  elephant,  only  the  miserly  tree  locks 
up  the  sunlight  in  solid  carbon. 

At  present,  any  boundary  of  the  animal  kingdom  is  ar- 
bitrary. "  We  cannot  distinguish  the  vegetable  from  the 
animal  kingdom  by  any  complete  and  precise  definition. 
Although  ordinary  observation  of  their  usual  representa- 
tives may  discern  little  that  is  common  to  the  two,  yet 
there  are  many  simple  forms  of  life  which  hardly  rise 
high  enough  in  the  scale  of  being  to  rank  distinctively 
either  as  plant  or  animal;  there  are  undoubted  plants 
possessing  faculties  which  are  generally  deemed  charac- 
teristic of  animals  ;  and  some  plants  of  the  highest  grade 
share  in  these  endowments."67 


CHAPTER   VI 

RELATION  BETWEEN  MINERALS,   PLANTS,   AND 
ANIMALS 

THERE  are  no  independent  members  of  creation :  all 
things  touch  upon  one  another.  The  matter  of  the  liv- 
ing world  is  identical  with  that  of  the  inorganic.  The 
plant,  feeding  on  the  minerals,  carbon  dioxide,  water, 
and  ammonia,  builds  them  up  into  complex  organic  com- 
pounds, as  starch,  sugar,  gum,  cellulose,  albumen,  and 
gluten.  When  the  plant  is  eaten  by  the  animal,  these 
substances  are  used  for  building  up  tissues,  supplying 
energy,  repairing  waste,  laid  up  in  reserve  as  glycogen 
and  fat,  or  oxidized  in  the  tissues  to  produce  heat.  The 
albuminoids  are  essential  for  the  formation  of  tissues, 
like  muscle,  nerve,  cartilage ;  the  ternary  compounds 
help  in  repairing  waste,  while  both  produce  heat.  When 
oxidized,  whether  for  work  or  warmth,  these  complex 
compounds  break  up  into  the  simple  compounds,  — 
water,  carbon  dioxide,  and  (ultimately)  ammonia,  and  as 
such  are  returned  to  earth  and  air  from  the  animal. 
Both  plant  and  animal  end  their  life  by  going  back 
to  the  mineral  world :  and  thus  the  circle  is  complete 
—  from  dust  to  dust.  Plants  compress  the  forces  of 
inorganic  nature  into  chemical  compounds ;  animals 
liberate  them.  Plants  produce;  animals  consume.  The 
work  of  plants  is  synthesis,  a  building-up ;  the  work 
of  animals  is  analysis,  or  destruction.  Without  plants, 
animals  would  perish ;  without  animals,  plants  had  no 
need  to  be. 

224 


CHAPTER   VII* 

LIFE 

ALL  forces  are  known  by  the  phenomena  which  they 
cause.  So  long  as  the  animal  and  plant  were  supposed 
to  exist  in  opposition  to  ordinary  physical  forces  or  inde- 
pendently of  them,  a  vital  force  or  principle  was  postu- 
lated by  which  the  work  of  the  body  was  performed. 
It  is  now  known  that  most,  if  not  all,  of  the  phenomena 
manifested  by  a  living  body  are  due  to  one  or  more  of 
the  ordinary  physical  forces,  —  heat,  chemical  affinity, 
electricity,  etc.  There  is  no  work  done  which  demands 
a  vital  force. 

The  common  modern  view  is  that  vitality  is  simply  a 
collective  name  for  the  sum  of  the  phenomena  displayed 
by  living  beings.  It  is  neither  a  force  nor  a  thing  at 
all,  but  is  an  abstraction,  like  goodness  or  sweetness ; 
or,  to  use  Huxley's  expression,  to  speak  of  vitality  is  as 
if  one  should  speak  of  the  horologity  of  a  clock,  mean- 
ing its  time-keeping  properties. 

A  third  theory  is  still  possible.  The  combination  of 
elements  into  organic  cells,  the  arrangement  of  these 
cells  into  tissues,  the  grouping  of  these  tissues  into 
organs,  and  the  marshaling  of  these  organs  into  plans 
of  structure,  call  for  some  further  shaping,  controlling 
power  to  effect  such  wonderful  coordination.  More- 
over, the  manifestation  of  feeling  and  consciousness  is  a 
mystery  which  no  physical  hypothesis  has  cleared  up. 
The  simplest  vital  phenomenon  has  in  it  something  over 

*  See  Appendix. 
DODGE'S  GEN.  ZOOL.  —  15        225 


226  COMPARATIVE   ZOOLOGY 

and  above  the  known  forces  of  the  laboratory.68  If  the 
vital  machine  is  given,  it  works  by  physical  forces ;  but 
to  produce  it  and  keep  it  in  order  needs,  so  far  as  we 
now  know,  more  than  mere  physical  force.  To  this 
controlling  power  we  may  apply  the  name  vitality. 

Life  is  exhibited  only  under  certain  conditions.  One 
condition  is  the  presence  of  a  physical  basis  called  proto- 
plasm. This  substance  is  found  in  all  living  bodies, 
and,  so  far  as  we  know,  is  similar  in  all  —  a  viscid, 
transparent,  homogeneous,  or  minutely  granular,  albu- 
minoid matter.  Life  is  inseparable  from  this  proto- 
plasm ;  but  it  is  dormant  unless  excited  by  some 
external  stimulants,  such  as  heat,  light,  electricity,  food, 
water,  and  oxygen.  Thus,  a  certain  temperature  is 
essential  to  growth  and  motion;  taste  is  induced  by 
chemical  action,  and  sight  by  luminous  vibrations. 

The  essential  manifestations  of  animal  life  may  be 
reduced  to  four  :  contractility  ;  irritability,  or  the  power 
of  receiving  and  transmitting  impressions  ;  the  power  of 
assimilating  food ;  the  power  of  reproduction.  All  these 
powers  are  possessed  by  protoplasm,  and  so  by  all  ani- 
mals :  all  move,  feel,  grow,  and  multiply.  But  some  of 
the  lowest  forms  are  without  any  other  trace  of  organs 
than  is  found  in  a  simple  cell ;  they  seem  to  be  almost  as 
homogeneous  and  structureless  as  a  drop  of  jelly.  They 
could  not  be  more  simple.  They  are  devoid  of  muscles, 
nerves,  and  stomach  ;  yet  they  have  all  the  fundamental 
attributes  of  life,  —  moving,  feeling,  eating,  and  propa- 
gating their  kind.  The  animal  series,  therefore,  begins 
with  forms  that  feel  without  nerves,  move  without  mus- 
cles, and  digest  without  a  stomach,  protoplasm  itself 
having  all  these  properties :  in  other  words,  life  is  the 
cause  of  organization,  not  the  result  of  it.  Animals  do 
not  live  because  they  are  organized,  but  are  organized 
because  they  are  alive. 


CHAPTER   VIII* 
ORGANIZATION 

WE  have  seen  that  the  simplest  living  thing  is  a 
formless  speck  of  protoplasm,  without  distinctions  of 
structure,  and  therefore  without  distinctions  of  function, 
all  parts  serving  all  purposes  —  mouth,  stomach,  limb, 
and  lung  —  indiscriminately.  There  is  no  separate 
digestive  cavity,  no  separate  respiratory,  muscular,  or 
nervous  systems.  Every  part  will  successively  feed, 
feel,  move,  and  breathe.  Just  as  in  the  earliest  state  of 
society  all  do  everything,  each  does  all.  Every  man  is 
his  own  tailor,  architect,  and  lawyer.  But  in  the  prog- 
ress of  social  development  the  principle  of  the  division 
of  labor  emerges.  First  comes  a  distinction  between 
the  governing  and  governed  classes;  then  follow  and 
multiply  the  various  civil,  military,  ecclesiastical,  and 
industrial  occupations. 

In  like  manner,  as  we  advance  in  the  animal  series, 
we  find  the  body  more  and  more  heterogeneous  and 
complex  by  a  process  of  differentiation,  i.e.,  setting 
apart  certain  portions  of  the  body  for  special  duty.  In 
the  lowest  forms,  the  work  of  life  is  carried  on  by  very 
simple  apparatus.  But  in  the  higher  organisms  every 
function  is  performed  by  a  special  organ.  For  example, 
contractility,  at  first  the  property  of  the  entire  animal, 
becomes  centered  in  muscular  tissue  ;  respiration,  which 
in  simple  beings  is  effected  by  the  whole  surface,  is 
specialized  in  lungs  or  gills ;  sensibility,  from  j 

*  See  Appendix. 
227 


228 


COMPARATIVE   ZOOLOGY 


common  to  the  whole  organism,  is  handed  over  to  the 
nerves.  An  animal,  then,  whose  body,  instead  of  being 
uniform  throughout,  is  made  up  of  different  parts  for  the 
performance  of  particular  functions,  is  said  to  be  organ- 
ized. And  the  term  is  as  applicable  to  the  slightly  dif- 
ferentiated cell  as  to  complex  man.  Organization  is 
expressed  by  single  cells,  or  by  their  combination  into 
tissues  and  organs. 

i.  Cells.  —  A  cell  is  the  simplest  form  of  organized 
life.     In  general,  it  is  a  microscopic  globule,  consisting 


FIG.  igB.  —  A,  diagram  of  a  cell;  TV,  cell  wall,  with  inclosed  cytoplasm;  «,  nucleus, 
consisting  of  nuclear  membrane  inclosing  granular  substance,  in  which  are  seen  a 
spherical  nucleolus  and  several  irregular  masses  of  chromatin  ;  a,  attraction  sphere 
containing  a  centrosome.  B-F,  changes  which  take  place  in  the  cell  during  fission. 

of  a  delicate  membrane  inclosing  a  minute  portion  of 
protoplasm.  The  very  simplest  kinds  are  without  gran- 
ules or  signs  of  circulation ;  but  usually  the  protoplasm 
is  granular,  and  contains  a  defined  separate  mass  called 
the  nucleus,  within  which  are  sometimes  seen  one  or 
two,  rarely  more,  dark,  round  specks,  named  nucleoli. 
The  enveloping  membrane  is  extremely  thin,  transpar- 
ent, and  structureless ;  it  is  only  an  excretion  of  dead 
matter  acting  as  a  boundary  to  the  cell  contents.69  The 
nucleus  generally  lies  near  the  center  of  the  protoplasm, 
and  is  the  center  of  activity, 


ORGANIZATION  22Q 

Cells  vary  greatly  in  size,  but  are  generally  invisible 
to  the  naked  eye,  ranging  from  ^J7  to  IQTFO  °f  an  mcn 
in  diameter.  About  4000  of  the  smallest  would  be 
necessary  to  cover  the  dot  of  this  letter  i.  The  natural 
form  of  isolated  cells  is  spherical ;  but  when  they  crowd 
each  other,  as  seen  in  bone,  cartilage,  and  muscle,  their 
outlines  become  angular,  either  hexagonal  or  irregular. 

Within  the  narrow  boundary  of  a  simple  sphere,  the 
cell  membrane,  are  exhibited  all  the  essential  phenomena 
of  life,  —  nutrition,  sensation,  development,  and  repro- 
duction. The  physiology  of  these  minute  organisms  is 
of  peculiar  interest,  since  all  animal  structure  is  but  the 
multiplication  of  the  cell  as  a  unit,  and  the  whole  life  of 
an  animal  is  that  of  the  cells  which  compose  it :  in  them 
and  by  them  all  its  vital  processes  are  carried  on.70 

The  structure  of  an  animal  cell  can  be  seen  in  blood 
corpuscles,  by  diluting  with  a  weak  (.6  per  cent)  solu- 
tion of  salt  a  drop  of  blood  from  a  frog,  and  placing  it 
under  the  microscope.  (See  Fig.  260.)  With  this  may 
be  compared  vegetable  cells  as  seen  in  a  drop  of  fluid 
yeast  or  a  drop  of  water  into  which  pollen  grains  from 
some  flower  have  been  dusted. 

2.  Tissues.  —  There  are  organisms  of  the  lowest 
grade  (as  Paramecium,  Fig.  9)  which  consist  of  a  single 
cell,  living  for  and  by  itself.  In  this  case,  the  animal 
and  cell  are  identical :  the  Paramecium  is  as  truly  an 
individual  as  the  elephant.  But  all  animajs,  save  these 
unicellular  beings,  are  mainly  aggregations  of  cells ;  for 
the  various  parts  of  a  body  are  not  only  separable  by 
the  knife  into  bones,  muscles,  nerves,  etc.,  but  these  are 
susceptible  of  a  finer  analysis  by  the  microscope,  which 
shows  that  they  arise  from  the  development  and  union 
of  cells.  These  cellular  fabrics,  called  tissties,  differ 
fro/n  one  another  both  chemically  and  structurally,  but 
agree  in  being  permeable  to  liquids  —  a  property  which 


230  COMPARATIVE   ZOOLOGY 

secures  the  flexibility  of  the  organs  so  essential  to  ani- 
mal life.  Every  part  of  the  human  body,  for  example, 
is  moist ;  even  the  hairs,  nails,  and  cuticle  contain  water. 
The  contents  as  well  as  the  shape  of  the  cells  are  usually 
modified  according  to  the  tissue  which  they  form  :  thus, 
we  find  cells  containing  earthy  matter,  iron,  fat,  mucus, 
etc. 

In  plants,  the  cell  generally  retains  its  characters  well 
defined;  but  in  animals  (after  the  embryonic  period) 
the  cell  usually  undergoes  such  modifications  that  its 
structural  features  become  altered.  The  cells  are  con- 
nected together  or  enveloped  by  an  intercellular  sub- 
stance (matrix),  which  may  be  watery,  soft,  and 
gelatinous,  firmer  and  tenacious,  still  more  solid  and 
hyaline,  or  hard  and  opaque.  In  the  fluids  of  the  body, 
as  the  blood,  the  cells  are  separate ;  i.e.,  the  matrix  is 
fluid.  But  in  the  solid  tissues  they  are  held  together 
by  intercellular  substance. 

In  the  lowest  forms  of  life,  and  in  all  the  higher 
animals  in  their  earliest  embryonic  state,  the  cells  of 
which  they  are  composed  are  not  transformed  into 
differentiated  tissues :  definite  tissues  make  their  first 
appearance  in  the  sponges,  and  they  differ  from  one 
another  more  and  more  widely  as  we  ascend  the  scale 
of  being.  In  other  words,  the  bodies  of  the  lower  and 
the  immature  animals  are  more  uniform  in  composition 
than  the  higher  or  adult  forms.  In  the  vertebrates  only 
are  all  the  following  tissues  found  represented :  — 

(i)  Epithelial  Tissue. — This  is  the  simplest  form  of  cel- 
lular structure.  It  covers  all  the  free  surfaces  of  the 
body,  internal  and  external,  so  that  an  animal  maybe 
said  to  be  contained  between  the  walls  of  a  double  bag. 
That  which  is  internal,  lining  the  mouth,  windpipe, 
lungs,  blood  vessels,  gullet,  stomach,  intestines  —  in  fact, 
every  cavity  and  canal  —  is  called  epithelium.  It  is  a 


ORGANIZATION 


231 


very  delicate  skin,  formed  of  flat  or  cylindrical  cells, 
and  in  some  parts  (as  in  the  windpipe  of  air-breathing 
animals,  and  along  the  gills  of  the  oyster)  is  covered 
with  cilia,  or  minute  hairlike  portions  of  protoplasm, 
about  -$-^0-$  of  an  inch  long,  which  are  incessantly  mov- 
ing. Continuous  with  this  inner  lining  of  the  body  (as 
seen  on  the  lip),  and  covering  the  outside,  is  the  epi- 
dermis or  ctiticle.  It  is  the  outer  layer  of  the  "  skin," 
which  we  can  remove  by  a  blister,  and  in  man  varies  in 
thickness  from  -g~J7  of  an  inch  on  the  cheek  to  •£$  on 
the  sole  of  the  foot. 
It  is  constantly 
wearing  off  at  the 
surface,  and  as  con- 
stantly being  re- 
plenished from  the 
deeper  portion;  and 
in  the  process  of 
growth  and  pas- 
sage OUtward,  the  FlG  I99>_ Various  kinds  of  Epithelium  Cells  magnified; 
Cells  change  from  a'  c°lumnar»  from  small  intestine;  3,  a  single  cell, 

the  spherical  form 
to  dead  horny 
scales  (seen  in  scurf 
and  dandruff).  In  the  lower  layer  of  the  cuticle  we  find 
the  pigment  cells,  characteristic  of  colored  races.  Nei- 
ther the  epidermis  nor  the  corresponding  internal  tissue 
(epithelium)  has  any  blood  vessels  or  nerves.  The  epi- 
thelial tissue,  then,  is  simply  a  superficial  covering, 
bloodless  and  insensible,  protecting  the  more  delicate 
parts  underneath,  or,  as  in  the  alimentary  canal,  pro- 
ducing mucus  and  digestive  juices.  Hairs,  horns,  hoofs, 
nails,  claws,  corns,  beaks,  scales,  tortoise  shell,  the  wings 
of  insects,  etc.,  are  modifications  of  the  epidermis. 

The  next  three  sorts  of  tissue  are  .characterized  by  a 


showing  nucleus;  b,  ciliated,  from  one  of  the  small  air- 
tubes  ;  d,  the  same,  from  the  windpipe,  with  single  cell 
magnified,  about  200  times;  c,  squamous,  from  eyelid 
of  a  calf,  showing  changes  of  form,  from  the  deep  to 
superficial  cells,  i  being  the  scurf. 


232 


COMPARATIVE   ZOOLOGY 


great  development  of  the  intercellular  substance,  while 
the  cells  themselves  are  very  slightly  modified. 

(2)  Connective  Tissue.  —  This  is  the  most  extensive  tissue 
in  animals,  as  it  is  the  great  connecting  medium   by 

which  the  differ- 
ent parts  are  held 
together.  Could 
it  be  taken  out 
entire,  it  would 
be  a  complete 
mold  of  all  the 
organs.  It  sur- 
rounds the  bones, 
muscles,  blood 
vessels,  nerves, 
of  the  ligaments 


FIG.  200.  —  Connective  Tissue,  showing  areolar  structure. 

and   glands,    and   is   the   substance 


and  tendons,  and  forms  a  large  portion  of  "true  skin," 


FIG.  201.  —  Connective  Tissue  from  human  peritoneum;  highly  magnified;  a,  blood  vessel 


ORGANIZATION 


233 


mucous  membrane,  etc.  It  varies  in  character,  being 
soft,  tender,  and  elastic,  or  dense,  tough,  and  generally 
unyielding.  In  the  former  state,  it  consists  of  innu- 
merable fine  white  and  yellow  fibers,  which  interlace 
in  all  directions,  leaving  irregular  spaces,  and  forming  a 
loose,  spongy,  moist  web.  In  the  latter  the  fibers  are 
condensed  into  sheets  or  parallel  cords,  having  a  wavy, 
glistening  appearance.  Such  structures  are  the  fasciae 
and  tendons.  Connective  tissue  is  not  very  sensitive. 
It  contains  gelatin  —  the  matter  which  tans  when  hide 
is  made  into  leather.  In  this  tissue  the  intercellular 

substances    take    the 

form  of  fibers.     The 

white  fibers  are  ine- 

lastic, and  from 

to  °f  an 


diameter. 


FIG.  202.  -Hyaline  Car- 
tilage,  Diagram  :  a,  car- 

tilage cell;  t>,  ceil  about  dons. 

to  divide;  c,  cell  divided     _.  , 

into  two;  d,  into  four  fibers  are  elastic,  very 


They  are 

^Q    ten. 

The    yellow 


long,  and  from  „$„ 

to  "V  of  an  inch 

highly  magnified.         in     diameter,    and 
branched.       Connec- 
tive tissue  appears  areolar,  i.e.,  shows 
interspaces,  only  under  the  microscope. 

(3)  Cartilaginous  Tissue.  —  This  tissue,   FIG.  203.  —  Longitudinal 

.  .  section  through  area  of 

known  also  as  "  gristle,  is  composed  ossification  from  long 
of  cells  embedded  in  a  granular  or  bone  of  human  embryo. 
hyaline  substance,  which  is  dense,  elastic,  bluish  white, 
and  translucent.  It  is  found  where  strength,  elasticity, 
and  insensibility  are  wanted,  as  at  the  joints.  It  also 
takes  the  place  of  the  long  bones  in  the  embryo.  When 
cartilage  is  mixed  with  connective  tissue,  as  in  the  ear, 
it  is  called  fibre-cartilage. 


234 


COMPARATIVE   ZOOLOGY 


FIG.  204. — Transverse  section  of  a  Bone  (Human 
Femur),  x  50,  showing  Haversian  canals. 


(4)  Osseous  Tissue.  —  This  hard,  opaque  tissue,  called 
bone,  differs  from  the  former  two  in  having  the  inter- 
cellular spaces  or 
meshes  filled  with 
phosphate  of  lime 
and  other  earths,  in- 
stead of  a  hyaline  or 
fibrous  substance.  It 
may  be  called  pet- 
rified tissue  —  the 
quantity  of  earthy 
matter,  and  therefore 
the  brittleness  of  the 
bone,  increasing  with 
the  age  of  the  animal. 
If  a  chicken  bone  be 
left  in  dilute  muriatic  acid  several  days,  it  may  be  tied 
into  a  knot,  since  the  acid  has  dissolved  the  lime,  leav- 
ing nothing  but  cartilage  and  connective  tissue.  If  a 
bone  be  burned,  it  be- 
comes light,  porous, 
and  brittle,  the  lime 
alone  remaining.71 

Bone  is  a  very  vas- 
cular tissue;  that  is, 
it  is  traversed  by 
minute  blood  vessels 
and  nerves,  which 
pass  through  a  net- 
work of  tubes,  called 
Haversian  canals. 

The     Canals     average     FjG  2Q5  _  Frontai  Bone  of  Human  Skull  under  the 
__.!_._    Q£    g^    inch      be-  microscrope,  showing  lacunae  and  canaliculi. 

ing  finest  near  the    surface    of   the   bone,  and  larger 
farther  in,  where  they  form  a  cancellated  or  spongy 


ORGANIZATION 


235 


structure,  and  finally  merge  (in  the  long  bones)  into  the 
central  cavity  containing  the  marrow.  Under  the  micro- 
scope, each  canal  appears  to  be  the  center  of  a  multitude 
of  lamince,  or  plates,  arranged  around  it.  Lying  between 
these  plates  are  little  cavities,  called  lacunce,  which  are 
connected  by  exceedingly  fine  tubes,  or  canaliculi.  The 
two  represent  the  spaces  occupied  by  the  original  cells 
of  the  bone,  and  differ  in  shape  and  size  in  different 
animals. 

True  bone  is  found  only  in  vertebrates,  or  backboned 
animals. 

(5)  Dental  Tissue.  —  Like  bone,  a  tooth  is  a  combination 
of  earthy  and  animal  matter.  It  may  be  called  petrified 
skin.  In  the  higher  animals,  it  consists  of  three  parts : 


FIG.  206.  —  Highly  magnified  section  of  Dentine  and  Cement,  from  the  fang  of  a  Human 
Molar:  a,  b,  marks  of  the  original  dentinal  pulp;  d,  dentinal  tubes,  terminating  in 
the*  very  sensitive,  modified  layer,  g;  hy  cement. 

dentine,  forming  the  body  of  the  tooth,  and  always  pres- 
ent; enamel,  capping  the  crown;  and  cement,  covering 
the  fangs  (Fig.  229).  The  last  is  true  bone,  or  osseous 
tissue.  Dentine  resembles  bone,  but  differs  in  having 
neither  lacunae  nor  (save  in  shark's  teeth)  canaliculi.  It 
shows,  in  place  of  the  former,  innumerable  parallel  tubes, 
reaching  from  the  outside  to  the  pulp  cavity  within. 
The  "  ivory  "  of  elephants  consists  of  dentine.  Enamel 
is  the  hardest  substance  in  the  body,  and  is  composed 
of  minute  six-sided  fibers,  set  closely  together.  It  is 


236  COMPARATIVE   ZOOLOGY 

wanting  in  the  teeth  of  most  fishes,  snakes,  sloths, 
armadillos,  sperm  whales,  etc. 

True  dental  tissue  is  confined  to  vertebrates. 

(6)  Adipose  Tissue.  —  Certain  cells  become  greatly  en- 
larged and  filled  with  fat,  so  that  the  original  protoplasm 
occupies  a  very  small  part  of  the  space  within  the  cell 
membrane.  These  cells  are  united  into  masses  by  con- 
nective tissue,  in  the  skin  (as  in  the  "  blubber  "  of  whales), 
between  the  muscles  (as  in  "  streaky "  meat),  or  in 


FIG.  207.  —  Fat  Cells  embedded  in  Subcutaneous  Areolar  Tissue.  _/,  fat  cells;  «.  nucleus; 
c,  connective-tissue  corpuscles;  w,  migratory  cells;  e,  elastic  fibers;  b,  capillary 
blood  vessel. 

the  abdominal  cavity,  in  the  omentum,  mesentery,  or 
about  the  kidneys.  The  marrow  of  bones  is  an  example. 
Globules  of  fat  occur  in  many  mollusks  and  insects ;  but 
true  adipose  tissue  is  found  only  in  backboned  animals, 
particularly  in  the  herbivorous.  In  the  average  man, 
it  constitutes  about  £$  part  of  his  weight,  and  a  single 
whale  has  yielded  120  tons  of  oil.  The  fat  of  animals 
has  the  different  names  of  oil,  lard,  tallow,  suet,  sperma- 
ceti, etc.  It  is  a  reserve  of  nutriment  in  excess  of  con- 
sumption, serving  also  as  a  packing  material,  and  as  a 
protection  against  cold. 

(7)  Muscular  Tissue.  —  If  we  examine  a  piece  of  lean 
meat,  we  find  it  is  made  up  of  a  number  of  fasciculi,  or 
bundles  of  fibers,  placed  side  by  side,  and  bound  together 


ORGANIZATION 


237 


by  connective  tissue.     The  microscope  informs  us  that 

each  fiber  is  itself  a  bundle  of  smaller  fibers;  and  when 

one  of  these  is  more  closely  exam- 

ined,   it   is   found   to    consist   of   a 

delicate,  smooth  tube,  called  the  sar- 

co  lemma,  which  is  filled  with    very 

minute,    parallel     fibrils,    averaging 

lowo  °f  an  m°h  m  diameter,  the 

whole  having  a  striated  aspect  and 

containing  numerous  nuclei.    Tissue 

of    this   description    constitutes   all 

ordinary    muscle,    or  "lean    meat," 

and    is    marked    by    regular    cross    FlG 

lines,  or  s  trice. 

Besides  this  striated 

muscular  tissue,  there 

exist,  in  the   coats  of 

the  stomach,  intestines, 

blood  vessels,  and  some 

other   parts   of    verte- 

brates, smooth  muscular  fibers,  which  show 

a  single  nucleus  under  the  microscope,  and 

do  not  break  up  into   fibrils  (Fig.   319). 

The  gizzards  of  fowls  exhibit  this  form. 
All  muscle  has  the  property  of  shorten- 

ing itself  when  excited;  but  the  contraction 

of  the  striated  kind  is  under  the  control  of 

the  will,  while  the  movement  of  the  smooth 
™       -  fibers  is  involuntary.72     Muscles  are  well 

Man,  divided  by  * 

transverse  septa  supplied  with  arteries,  veins,  and  nerves  ; 

into  separate  nu-    ,  ,  .  .  .  ' 

portions;  but  the  color  is  due  to  a  peculiar  pigment, 


208.  —  Voluntary 
Muscle,  portions  of  two 
fibers  showing  the  char- 
acteristic transverse  mark- 
ings; the  lighter  band  is 
divided  by  the  row  of 
minute  beads  constituting 
the  intermediate  disk: 
a,  termination  of  muscular 
substance  and  attachment 
of  adjoining  fibrous  tissue; 
«,  nuclei  of  muscle  fibers. 


much    magnified. 

Muscular  tissue  is  found  in  all  animals  from  the  coral 
to  man. 

(8)  Nervous  Tissue.  —  Nervous  Tissue  consists  of  large, 


238 


COMPARATIVE   ZOOLOGY 


nucleated  cells,  which  give  off  one  to  several  processes, 
the  latter  serving  as  paths  of  communication  between 
the  cells  themselves  or  between  the  cells  and  the  various 
motor,  sensory,  and  other  organs  with  which  they  are 
connected.  Such  threads  of  nerve  tissue  are  called 


R 


[R 


FIG.  210. —  Portion 
of  two  Nerve 
Fibers:  a,  medul- 
lary sheath;  c,  nu- 
cleus of  neuri- 
lemma;  R,  annu- 
lar constriction. 


Dendrites 

Nerve  process 
Collateral  branch 


Axis  cylinder 


Neurilemma 


Terminal  branches 


FIG.  211.  —Diagram  of  a  Neuron:  a,  nerve  pro- 
cess; b,  neurilemma;  c,  medullary  sheath; 
d,  neurilemma  and  medullary  sheath,  combined. 


nerve  fibers,  and  each  consists  essentially  of  a  prolonga- 
tion of  the  protoplasrnic  substance  of  which  the  cell 
body  is  composed.  The  cells  vary  from  35*5-0  to  %^Q  of 
an  inch  in  diameter,  and  are  found  in  the  nerve  centers 
(Fig.  329),  the  gray  portion  of  the  brain,  spinal  cord, 
and  other  ganglia.  The  fibers  vary  in  structure.  In 
the  lowest  Metazoa  they  are  merely  naked  threads  of 


ORGANIZATION  239 

protoplasm.  In  the  higher  animals  each  thread,  known 
as  the  axis  cylinder,  1s  surrounded  by  a  delicate,  trans- 
parent covering  called  the  neurilemma,  analogous  to  the 
sarcolemma  of  muscle  tissue.  In  the  vertebrates,  the 
protoplasmic  threads  found  in  many  parts  of  the  nervous 
system  have  an  additional  covering  made  of  fatty 
material,  which  lies  between  the  axis  cylinder  and  the 
neurilemma,  and  is  known  as  the  medullary  sheath. 
These  are  called  medullated  nerve  fibers,  as  distinguished 


FIG.  212.  —  Spinal  Ganglion,  in  longitudinal  section,  from  Cat;  the  groups  of  nerve  cells 
lie  embedded  among  the  bundles  of  the  nerve  fibers. 


from  the  nonmedullated  or  those  which  lack  the  medul- 
lary sheath.  Fibers  of  the  former  kind  are  found  in  the 
white  substance  of  the  brain  and  spinal  cord,  and  run 
to  the  muscles  and  organs  of  sense.  Nonmedullated 
fibers  are  found  in  the  gray  substance  of  the  nervous 
system.  The  axis  cylinders  are  destitute  of  a  sheath  in 
the  neighborhood  of  the  cell  body.  Scattered  along  the 
fibers  nuclei  are  found.  The  large  nerve  fibers  may  be 
T2Vo  of  an  inch  in  diameter,  and  some  are  supposed  to 
extend  from  cell  bodies  situated  in  the  lower  part  of  the 
spinal  cord  down  the  leg  to  the  foot. 

A  bundle  of  nerve  fibers  surrounded  by  connective 
tissue  constitutes  a  nerve  in  the  anatomical  sense. 


240  COMPARATIVE   ZOOLOGY 

3.  Organs  and  their  Functions.  —  Animals,  like  plants, 
grow,  feel,  and  move ;  these  three  are  the  capital  facts 
of  every  organism.  Besides  these  there  may  be  some 
peculiar  phenomena,  as  motion  and  will. 

Life  is  manifested  in  certain  special  operations, 
called  functions,  performed  by  certain  special  parts, 
called  organs.  Thus,  the  stomach  is  an  organ,  whose 
function  is  digestion.  A  single  organ  may  manifest 
vitality,  but  it  does  not  (save  in  the  very  lowest  forms) 
show  forth  the  whole  life  of  the  animal.  For,  in  being 
set  apart  for  a  special  purpose,  an  organ  takes  upon 
itself,  so  to  speak,  to  do  something  for  the  benefit  of 
the  whole  animal,  in  return  for  which  it  is  absolved 
from  doing  many  things.  The  stomach  is  not  called 
upon  to  circulate  or  purify  the  blood. 

There  may  be  functions  without  special  organs,  as 
the  amoeba  digests,  respires,  moves,  and  reproduces  by 
its  general  mass.  But,  as  we  ascend  the  scale  of  animal 
life,  we  pass  from  the  simple  to  the  complex :  groups 
of  cells  or  tissues,  instead  of  being  repetitions  of  each 
other,  take  on  a  difference,  and  become  distinguished 
as  special  parts  with  specific  duties.  The  higher  the 
rank  of  the  animal,  the  more  complicated  the  organs. 
The  more  elaborated  the  structure,  the  more  compli- 
cated the  functions.  But  in  all  animals,  the  functions 
are  performed  under  conditions  essentially  the  same. 
Thus,  respiration  in  the  sponge,  the  fish,  and  in  man 
has  one  object  and  one  means,  though  the  methods 
differ.  A  function,  therefore,  is  a  group  of  similar 
phenomena  effected  by  analogous  structures. 

The  life  of  an  animal  consists  in  the  accumulation 
and  expenditure  of  force.  The  tissues  are  storehouses 
of  power,  which,  as  waste  goes  on,  is  given  off  in  various 
forms.  Thus,  the  nervous  tissue  generates  nerve  force; 
the  muscles,  motion.  If  we  contemplate  the  phenomena 


ORGANIZATION  241 

presented  by  a  dog,  the  most  obvious  fact  is  his  power 
of  moving  from  place  to  place,  a  power  produced  by 
the  interplay  of  muscles  and  bones.  We  observe,  also, 
that  his  motions  are  neither  mechanical  nor  irregular ; 
there  is  method  in  his  movement.  He  has  the  power 
of  willing,  seeing,  hearing,  feeling,  etc.  ;  and  these 
functions  are  accomplished  by  a  delicate  apparatus  of 
nerves. 

But  the  dog  does  not  exhibit  perpetual  motion. 
Sooner  or  later  he  becomes  exhausted,  and  rest  is 
necessary.  Sleep  gives  only  temporary  relief.  In 
every  exercise  of  the  muscles  and  nerves  there  is  a 
consumption  or  waste  of  their  substance.  The  blood 
restores  the  organs,  but  in  time  the  blood  itself  needs 
renewal.  If  not  renewed,  the  animal  becomes  emaci- 
ated, for  the  whale  body  is  laid  under  contribution  to 
furnish  a  supply.  Hence  the  feelings  of  hunger  and 
thirst,  impelling  the  creature  to  seek  food.  Only  this 
will  maintain  the  balance  between  waste  and  repair. 
We  notice,  therefore,  an  entirely  different  set  of  func- 
tions, involving,  however,  the  use  of  motion  and  will. 
The  dog  seizes  a  piece  of  meat,  grinds  it  between  his 
teeth  and  swallows  it.  It  passes  into  the  stomach, 
'where  it  is  digested,  and  then  into  the  intestine, 
where  it  is  further  changed;  there  the  nourishing 
part  is  absorbed,  and  carried  to  the  heart,  which 
propels  it  through  tubes,  called  blood  vessels,  all 
over  the  body.  In  this  process  of  digestion,  certain 
fluids  are  required,  as  saliva,  gastric  juice,  and  bile : 
these  are  secreted  by  special  organs,  called  glands. 
Moreover,  since  not  all  the  food  eaten  is  fitted  to  make 
blood,  and  as  the  blood  itself,  in  going  around  the  body, 
acts  like  a  scavenger,  picking  up  worn-out  particles,  we 
have  another  function,  that  of  excretion,  or  removal  of 
useless  matter  from  the  system^  The  kidneys  and  lungs 
DODGE'S  GEN.  ZOOL.  —  16 


242  COMPARATIVE   ZOOLOGY 

do  much  of  this ;  but  the  lungs  do  something  else. 
They  expose  the  blood  to  the  air,  and  introduce  oxygen, 
which,  we  shall  find,  is  essential  to  the  life  of  every 
animal. 

These  centripetal  and  centrifugal  movements  in  the 
body  —  throwing  in  and  throwing  out  —  are  so  related 
and  involved,  especially  in  the  lower  forms,  that  they 
can  not  be  sharply  defined  and  classified.  It  has  been 
said  that  every  dog  has  two  lives,  —  a  vegetative  -and  an 
animal.  The  former  includes  the  processes  of  digestion, 
circulation,  respiration,  secretion,  etc.,  which  are  com- 
mon to  all  life  ;  while  the  functions  included  by  the  latter, 
as  motion,  sensation,  and  will,  are  characteristic  of  animals. 
The  heart  is  the  center  of  the  vegetative  life,  and  the  brain 
is  the  center  of  the  animal  life.  The  aim  of  the  vegeta- 
tive organs  is  to  nourish  the  individual,  and  reproduce 
its  kind ;  the  organs  of  locomotion  and  sense  establish 
relations  between  the  individual  and  the  world  without. 
The  former  maintain  life ;  the  others  express  it.  The 
former  develop,  and  afterward  sustain,  the  latter.  The 
vegetative  organs,  however,  are  not  independent  of  the 
animal;  for  without  muscles  and  nerves  we  could  not 
procure,  masticate,  and  digest  food.  The  closer  the 
connection  and  dependence  between  these  two  sets  of 
organs,  the  higher  the  rank.73 

All  the  apparatus  and  phenomena  of  life  may  be  in- 
cluded under  the  heads  of  - 

NUTRITION, 
MOTION, 
SENSATION, 
REPRODUCTION. 

These  four  are  possessed  by  all  animals,  but  in  a 
variety  of  ways.  No  two  species  have  exactly  the  same 
mechanism  and  method  of  life.  We  must  learn  to  dis- 


ORGANIZATION  243 

tinguish  between  what  is  necessary  and  what  is  only  ac- 
cessory. That  only  is  essential  to  life  which  is  common 
to  all  forms  of  life.  Our  brains,  stomachs,  livers,  hands, 
and  feet  are  luxuries.  They  are  necessary  to  make  us 
human,  but  not  living,  beings.  Half  of  our  body  is 
taken  up  with  a  complicated  system  of  digestion ;  but 
the  amoeba  has  neither  mouth  nor  stomach.  We  have 
an  elaborate  apparatus  of  motion ;  the  adult  oyster  can 
not  stir  an  inch. 

Nutrition,  Motion,  and  Sensation  indicate  three  steps 
up  the  grade  of  life.  Thus,  the  first  is  the  prominent 
function  in  the  coral,  which  simply  "vegetates,"  the 
powers  of  moving  and  feeling  being  very  feeble.  In 
the  higher  insect,  as  the  bee,  there  is  great  activity  with 
simple  organs  of  nutrition.  In  the  still  higher  mam- 
mal, as  man,  there  is  less  power  of  locomotion,  though 
the  most  perfect  nutritive  system ;  but  both  functions 
are  subordinate  to  sensation,  which  is  the  crowning 
development. 

In  studying  the  comparative  anatomy  and  physiology 
of  the  animal  kingdom,  our  plan  will  be  to  trace  the 
various  organs  and  functions,  from  their  simplest  expres- 
sion upward  to  the  highest  complexity.  Thus  Nutrition 
will  begin  with  absorption,  which  is  the  simplest  method 
of  taking  food;  going  higher,  we  find  digestion,  but  in 
no  particular  spot  in  the  body ;  next,  we  see  it  confined 
to  a  tube ;  then  to  a  tube  with  a  sac,  or  stomach  ;  and, 
finally,  we  reach  the  complex  arrangement  of  the  higher 
animals. 


CHAPTER   IX 
NUTRITION 

Nutrition  is  the  earliest  and  most  constant  of  vital 
operations.  So  prominent  is  the  nutritive  apparatus, 
that  an  animal  has  been  likened  to  a  moving  sac,  organ- 
ized to  convert  foreign  matter  into  its  own  likeness,  to 
which  the  complex  organs  of  animal  life  are  but  auxil- 
iaries. Thus,  the  bones  and  muscles  are  levers  and  cords 
to  carry  the  body  about,  while  the  nervous  system  directs 
its  motions  in  quest  of  food. 

The  objects  of  nutrition  are  growth,  repair,  propaga- 
tion, and  supplying  energy  to  perform  the  work,  or 
functions,  of  the  body.  The  first  object  of  life  is  to 
grow,  for  no  animal  is  born  finished.  Some  animals, 
like  plants,  grow  as  long  as  they  live;  but  the  ma- 
jority soon  attain  a  fixed  size.  In  all  animals,  how- 
ever, without  exception,  food  is  wanted  for  another 
purpose  than  growth,  namely,  to  repair  the  waste 
which  is  constantly  going  on.  For  every  exercise  of 
the  muscles  and  nerves  involves  the  death  and  decay 
of  those  tissues,  as  shown  by  the  excretions.  The 
amount  of  matter  expelled  from  the  body,  and  the 
amount  of  nourishment  needed  to  make  good  the  loss, 
increase  with  the  activity  of  the  animal.  The  supply 
must  equal  the  demand,  in  order  to  maintain  the  life  of 
the  individual ;  and  as  an  animal  can  not  make  food, 
it  must  seek  it  from  without.  Not  only  the  muscles  and 
nerves  are  wasted  by  use,  but  every  organ  in  the  body ; 
so  that  the  whole  structure  needs  constant  renewal.  An 

244 


NUTRITION  245 

animal  begins  to  die  the  moment  it  begins  to  live.  The 
function  of  nutrition,  therefore,  is  constructive,  while 
motion  and  sensation  are  destructive. 

Another  source  of  demand  for  food  is  the  production 
of  germs,  to  propagate  the  race,  and  the  nourishment 
of  such  offspring  in  the  egg  and  infantile  state.  This 
reproduction  and  development  of  parts  which  can  main- 
tain an  independent  existence  is  a  vegetative  phenome- 
non (for  plants  have  it),  and  is  a  part  of  the  general 
process  of  nutrition.  But  it  will  be  more  convenient 
to  consider  it  hereafter  (Chapters  XXIL,  XXIII.).  Still 
another  necessity  for  aliment  among  the  higher  animals 
is  the  maintenance  of  bodily  heat.  This  will  be  treated 
under  the  head  of  Respiration. 

For  the  present,  we  will  study  nutrition,  as  mani- 
fested in  maintaining  the  life  of  an  adult  individual. 

In  all  animals,  this  process  essentially  consists  in  the 
introduction  of  food,  its  conversion  into  tissue,  its  oxida- 
tion, and  the  removal  of  worn-out  material. 

1.  The  food  must  be  procured,  and  swallowed.     (In- 
gestion.) 

2.  The  food  must  be  dissolved.     (Digestion.) 

3.  The  nutritive  fluid  must  be  taken  up,  and  then  dis- 
tributed all  over  the  body.     (Absorption  and  circulation.) 

4.  The  tissues  must  repair  their  parts  wasted  by  use, 
by  transforming  a  portion  of  the  blood  into  living  mat- 
ter like  themselves.     (Assimilation.) 

5.  Certain  matters  must  be  eliminated  from  the  blood, 
some  to  serve  a  purpose,  others  to  be  cast  out  of  the 
system.     (Secretion  and  excretion.) 

6.  In  order  to  produce  work  and  heat,  the  food  must 
be  oxidized,  either  in  the  blood  or  in  the  tissues,  after 
assimilation.     The  necessary  oxygen  is  obtained  through 
exposure  of  the  blood  to  the  air  in  the  lungs.     (Respira- 
tion in  part.) 


246  COMPARATIVE   ZOOLOGY 

7.  The  waste  products  of  this  oxidation  taken  up  by 
the  blood  must  be  got  rid  of ;  some  from  the  lungs  (car- 
bon dioxide,  water),  some  from  the  kidneys  (water,  urea, 
mainly),  some  from  the  skin  (water,  salines).  (Respira- 
tion in  part,  excretion.) 

The  mechanism  to  accomplish  all  this  in  the  lowest 
forms  of  life  is  exceedingly  simple,  a  single  cavity  and 
surface  performing  all  the  functions.  But  in  the  major- 
ity of  animals  the  apparatus  is  very  complicated  :  there 
is  a  set  of  organs  for  the  prehension  of  food ;  another 
for  digestion ;  a  third,  for  absorption ;  a  fourth,  for  dis- 
tribution ;  and  a  fifth,  for  purification. 


CHAPTER  X 

THE   FOOD   OF   ANIMALS 

THE  term  food  includes  all  substances  which  con- 
tribute to  nutrition  and  furnish  energy,  whether  they 
simply  assist  in  the  process,  or  are  actually  appropriated, 
and  become  tissue.  With  the  food  is  usually  combined 
more  or  less  indigestible  matter,  which  is  separated  in 
digestion. 

Food  is  derived  from  the  mineral,  vegetable,  and  ani- 
mal kingdoms.  Water  and  salt,  for  example,  are  inor- 
ganic. The  former  is  the  most  abundant,  and  a  very 
essential  article  of  food.  Most  of  the  lower  forms  of 
aquatic  life  seem  to  live  by  drinking :  their  real  nourish- 
ment, however,  is  present  in  the  water  in  the  form  of 
fine  particles.  The  earthworm,  some  beetles,  and  cer- 
tain savage  tribes  of  men  swallow  earth ;  but  this,  like- 
wise, is  for  the  organic  matter  which  the  earth  contains. 
As  no  animal  is  produced  immediately  from  inorganic 
matter,  so  no  animal  can  be  sustained  by  it. 

Nutritious  or  tissue-forming  food  comes  from  the 
organic  world,  and  is  albuminous,  as  the  lean  meat  of 
animals  and  the  gluten  of  wheat ;  oleaginous,  as  animal 
fat  and  vegetable  oil ;  or  saccharine,  as  starch  and  sugar. 
The  first  is  the  essential  food  stuff ;  no  substance  can 
serve  permanently  for  food  —  that  is,  can  permanently 
prevent  loss  of  weight  in  the  body  —  unless  it  contains 
albuminous  matter.  As  stated  before,  all  the  living 
tissues  are  albuminous,  and  therefore  albuminous  food 
is  required  to  supply  their  waste.  Albumen  contains 

247 


248  COMPARATIVE   ZOOLOGY 

nitrogen,  which  is  necessary  to  the  formation  of  tissue ; 
fats  and  sugars  are  rich  in  carbon,  and  therefore  serve 
to  maintain  the  heat  of  the  body,  and  to  repair  part  of 
the  waste  of  tissues.  Many  warm-blooded  animals  feed 
largely  on  farinaceous  or  starchy  substances,  which  in 
digestion  are  converted  into  sugar.  But  any  animal, 
of  the  higher  orders  certainly,  whether  herbivorous  or 
carnivorous,  would  starve  if  fed  on  pure  albumen,  oil, 
or  sugar.  Nature  insists  upon  a  mixed  diet ;  and  so  we 
find  in  all  the  staple  articles  of  food,  as  mil^,  meat,  and 
bread,  at  least  two  of  these  principles  present.  As  a 
rule,  the  nutritive  principles  in  vegetables  are  less  abun- 
dant than  in  animal  food,  and  the  indigestible  residue 
is  consequently  greater.  The  nutriment  in  flesh  in- 
creases as  we  ascend  the  animal  scale ;  thus,  oysters 
are  less  nourishing  than  fish ;  fish,  less  than  fowl ;  and 
fowl,  less  than  the  flesh  of  quadrupeds. 

Many  animals,  as  most  insects  and  mammals,  live 
solely  on  vegetable  food,  and  some  species  are  restricted 
to  particular  plants,  as  the  silkworm  to  the  white  mul- 
berry. But  the  majority  of  animals  feed  on  one  an- 
other; such  are  hosts  of  the  microscopic  forms,  and 
nearly  all  the  radiated  species,  marine  mollusks,  crusta- 
ceans, beetles,  flies,  spiders,  fishes,  amphibians,  reptiles, 
birds,  and  clawed  quadrupeds. 

A  few,  as  man  himself,  are  omnivorous,  i.e.,  are  main- 
tained on  a  mixture  of  animal  and  vegetable  food.  The 
use  of  fire  in  the  preparation  of  food  is  peculiar  to  man, 
who  has  been  called  "  the  cooking  animal."  A  few  of 
the  strictly  herbivorous  and  carnivorous  animals  have 
shown  a  capacity  for  changing  their  diet.  Thus,  the 
horse  and  cow  may  be  brought  to  eat  fish  and  flesh  ;  the 
sea  birds  can  be  habituated  to  grain ;  cats  are  fond  of 
alligator-pears  ;  and  dogs  take  naturally  to  the  plantain. 
Certain  animals,  in  passing  from  the  young  to  the 


THE   FOOD    OF  ANIMALS  249 

mature  state,  make  a  remarkable  change  of  food.  Thus, 
the  tadpole  feeds  upon  vegetable  matter ;  but  the  adult 
frog  is  carnivorous,  living  on  insects,  worms,  and 
crustaceans. 

Many  tribes,  especially  of  reptiles  and  insects,  are 
able  to  go  without  food  for  months,  or  even  years.  In- 
sects in  the  larval,  or  caterpillar,  state  are  very  vora- 
cious ;  but  upon  reaching  the  perfect,  or  winged,  state, 
they  eat  little  —  some  species  taking  no  food  at  all,  the 
mouth  being  actually  closed.  The  males  of  some  roti- 
fers and  other  tribes  take  no  food  from  the  time  of  leav- 
ing the  egg  until  death. 

In  general,  the  greater  the  facility  with  which  an  ani- 
mal obtains  its  food,  the  more  dependent  is  it  upon  a 
constant  supply.  Thus,  carnivores  endure  abstinence 
better  than  herbivores,  and  wild  animals  than  domesti- 
cated ones. 


CHAPTER   XI 

HOW   ANIMALS   EAT 

i 

i.  The  Prehension  of  Food.  —  (i)  Liquids.  —  The  sim- 
plest method  of  taking  nourishment,  though  not  the 
method  of  the  simplest  animals,  is  by  absorption  through 
the  skin.  The  tapeworm,  for  example,  living  in  the 
intestine  of  its  host,  has  neither  mouth  nor  stomach, 
but  absorbs  the  digested  food  with  which  its  body  is 
bathed  (Fig.  37).  Many  other  animals,  especially  in- 
sects, live  upon  liquid  food,  but  obtain  it  by  suction 
through  a  special  orifice  or  tube.  Thus,  we  find  a 
mouth,  or  sucker,  furnished  with  teeth  for  lancing  the 
skin  of  animals,  as  in  the  leech ;  a  bristlelike  tube  fitted 
for  piercing,  as  in  the  mosquito ;  a  sharp  sucker  armed 
with  barbs,  to  fix  it  securely  during  the  act  of  sucking, 
as  in  the  louse ;  and  a  long,  flexible  proboscis,  as  in  the 
butterfly  (Fig.  221).  Bees  have  a  hairy,  channeled 
tongue  (Fig.  220),  and  flies  have  one  terminating  in  a 
large,  fleshy  knob,  with  or  without  little  "  knives "  at 
the  base  for  cutting  the  skin  (Fig.  222);  both  lap, 
rather  than  suck,  their  food. 

Most  animals  drink  by  suction,  as  the  ox ;  and  a  few 
by  lapping,  as  the  dog ;  the  elephant  pumps  the  water 
up  with  its  trunk,  and  then  pours  it  into  its  throat ;  and 
birds  (excepting  doves)  fill  the  beak,  and  then,  raising 
the  head,  allow  the  water  to  run  down. 

Many  aquatic  animals,  whose  food  consists  of  small 
particles  diffused  through  the  water,  have  an  apparatus 
for  creating  currents,  so  as  to  bring  such  particles  within 

250- 


HOW  ANIMALS   EAT  251 

their  reach.  This  is  particularly  true  of  low,  fixed  forms, 
which  are  unable  to  go  in  search  of  their  food.  Thus, 
the  sponge  draws  nourishment  from  the  water,  which  is 
made  to  circulate  through  the  system  of  canals  travers- 
ing its  body  by  the  vibration  of  flagella,  lining  parts  of  the 
canals  (Figs.  14,  15).  The  microscopic  infusoria  have 
cilia  surrounding  the  mouth,  with  which  they  draw  or 
drive  into  the  body  little  currents  containing  nutritious 
particles  (Figs.  9,  n).  Bivalve  mollusks,  as  the  oyster 
and  clam,  are  likewise  dependent  upon  this  method  of 
procuring  food,  the  gills  and  inner  surface  of  the 
mantle  being  covered  with  cilia.  So  the  singular  fish, 
amphioxus  (the  only  example  among  vertebrates),  em- 
ploys ciliary  action  to  obtain  the  minute  organisms  on 
which  it  feeds  (Fig.  117).  The  Greenland  whale  has  a 
mode  of  ingestion  somewhat  unique,  gulping  great  vol- 
umes of  water  into  its  mouth,  and  then  straining  out, 
through  its  whalebone  sieve,  the  small  animals  which 
the  water  may  contain  (Fig.  171). 

(2)  Solids.  —  When  the  food  is  in  solid  masses,  whether 
floating  in  water  or  not,  the  animal  is  usually  provided 
with  prehensile  appendages 
for  taking  hold  of  it.  The 
jellylike  amoeba  (Fig.  i)  has 
neither  mouth  nor  stomach, 
but  extemporizes  them,  seiz- 
ing its  food  by  means  of  its 
soft  body.  The  food  then 
passes  through  the  denser, 

,        ,        FIG.   213.  —  A      Foraminifer      (Rotalia 
OUter      portion      OI       the      body        Veneta~),  with  pseudopodia  extended, 

into  the  softer  interior,  where 

it  is  digested.  The  waste  particles  are  passed  out  in 
the  reverse  direction.  In  the  foraminifers,  threadlike 
projections  (pseudopodia)  of  the  body  are  thrown  out 
which  adhere  to  the  prey.  The  soft  jellylike  substance 


252  COMPARATIVE   ZOOLOGY 

of  the  body  then  flows  toward  and  collects  about  the 
food,  and  digests  it  (Fig.  213). 

A  higher  type  is  seen  in  polyps  and  jellyfishes,  which 
have  hollow  tentacles  around  the  entrance  to  the  stomach 
(Figs.  1 8,  20,  26).  These  tentacles  are  contractile,  and 
some,  moreover,  are  covered  with  an  immense  number 
of  minute  sacs,  in  each  of  which  a  highly  elastic  filament 
is  coiled  up  spirally  (lasso  cells,  nettle  cells).  When 
the  tentacles  are  touched  by  a  passing  animal,  they 
seize  it,  and  at  the  same  mome~nt  throw  out  their  myriad 
filaments,  like  so  many  lassos,  which  penetrate  the  skin 
of  the  victim,  and  probably  also  emit  a  fluid,  which 
paralyzes  it ;  the  mouth,  meanwhile,  expands  to  an  ex- 
traordinary size,  and  the  creature  is  soon  ingulfed  in 
the  digestive  bag. 

In  the  next  stage,  we  find  no  tentacles,  but  the  food 
is  brought  to  the  mouth  by  the  flexible  lobes  of  the 
body  commonly  called  arms,  which  are  covered  with 
hundreds  of  minute  suckers  ;  and  if  the  prey,  as  an 
oyster,  is  too  large  to  be  swallowed,  the  stomach  pro- 
trudes, like  a  proboscis,  and  sucks  it  out  of  its  shell. 
This  is  seen  in  the  starfish  (Fig.  323). 

A  great  advance  is  shown  by  the  sea  urchin,  whose 
mouth  is  provided  with  five  sharp  teeth,  set  in  as  many 
jaws,  and  capable  of  being  projected  so  as  to  grasp, 
as  well  as  to  masticate,  its  food  (Figs.  48,  226). 

In  mollusks  having  a  single  shell,  as  the  snail,  the 
chief  organ  of  prehension  is  a  straplike  tongue,  covered 
with  minute  recurved  teeth,  or  spines,  with  which  the 
animal  rasps  its  food,  while  the  upper  lip  is  armed  with 
a  sharp,  horny  plate  (Fig.  227).  In  many  marine  species, 
as  the  whelk,  the  tongue  is  situated  at  the  end  of  a 
retractile  proboscis,  or  muscular  tube.  In  the  cuttlefish, 
we  see  the  sudden  development  of  an  elaborate  system 
of  prehensile  organs.  Besides  a  spinous  tongue,  it  has 


HOW  ANIMALS    EAT 


253 


a  pair  of  hard  mandibles,  resembling  the  beak  of  a 
parrot,  and  working  vertically ;  and  around  the  mouth 
are  eight  or  ten  powerful  arms 
furnished  with  numerous  cup- 
like  suckers.  So  perfect  is  the 
adhesion  of  these  suckers,  that 
it  is  easier  to  tear  away  a  limb 
than  to  detach  it  from  its  hold. 
The  earthworm  swallows 
earth  containing  particles  of 
decaying  vegetable  matter, 
which  it  secures  with  its  lips, 
the  upper  one  being  prolonged. 
Other  worms  (as  Nereis)  are 
so  constructed  that  the  gullet, 
which  is  frequently  armed  with 
teeth  and  forceps,  can  be  pro- 

FIG.  214.  —  Suckers  on  the  Tentacles 

truded  to  form  a  proboscis  for      Of  a  Cuttlefish:  «,  hoiiow  axis  of 
seizing  prey.  the  arm' containing  nerve  and  ar' 

The  Arthropoda  exhibit  a 
great  variety  of  means  for  pro- 
curing nourishment,  in  addi- 
tion to  the  suctorial  contrivances  already  mentioned,  the 
innumerable  modifications  of  the  mouth  corresponding 
to  the  diversity  of  food.  Mille- 
pedes, caterpillars,  and  grubs 
have  a  pair  of  horny  jaws  moving 
horizontally.  The  centipede  has 
a  second  pair  of  jaws,  which  are 
really  modified  feet,  terminated 
by  curved  fangs  containing  a 
,  poison  duct.  The  horseshoe  crab 

FIG.    215.  —  Nereis  —  head,     with  * 

extended    proboscis:     ?,   jaws;  US6S    its    feet    for  prehension,    aild 
T,  tentacles;  H,  head ;  £,  eyes.     -  ,   .    ,  .  ,     .     .  .   . 

the  thighs,  or  basal  joints  of  its 
legs,  to  masticate  the  food  and  force  it  into  the  stomach. 


tery;  c,  cellular  tissue;  d,  radiat- 
ing fibers;  h,  raised  margin  of  the 
disk  around  the  aperture  f,  g,  which 
contains  a  retractile  membrane,  or 
"  piston,"  i. 


254  COMPARATIVE  ZOOLOGY 

The  first  six  pairs  of  legs  in  the  lobster  and  crab  are 
likewise  appropriated  to  conveying  food  into  the  mouth, 
the  sixth  being  enormously  developed,  and  furnished 
with  powerful  pincers.  Scorpions  have  a  similar  pair 

of  claws  for  prehension,  and 
also  a  pair  of  small  forceps 
for  holding  the  food  in  con- 
tact with  the  mouth.  In 
their  relatives,  the  spiders, 
the  claws*  are  wanting,  and 
.  the  forceps  end  in  a  fang, 

*  IG.  216.  —  One  of  the  Fangs,  or  Perforated  r  °' 

Mandibles,  of  the  Spider,  much  magni-    Or  hook,  which  is  perforated 
fied.  7A 

to  convey  venom.'4 

The  biting  insects,  as  beetles  and  locusts,  have  two 
pairs  of  horny  jaws,  which  open  sidewise,  one  above  and 
the  other  below  the  oral  orifice.  The  upper  pair  are 
called  mandibles ;  the  lower,  maxillae.  The  former  are 
armed  with  sharp  teeth,  or  with  cutting  edges,  and 
sometimes  are  fitted,  like  the  molars  of  quadrupeds,  to 
grind  the  food.  The  maxillae  are  usually  composed  of 
several  parts,  some  of  which  serve  to  hold  the  food, 
or  to  help  in  dividing  it,  while  others  (palpi)  are  both 
sensory  and  prehensile.  There,  is  generally  present  a 
third  pair  of  jaws  —  the  labium  —  which  are  united  in 
the  middle  line,  and  serve  as  a  lower  lip.  They  also 
bear  palpi.  The  mantis  seizes  its  prey  with  its  long 
fore  legs,  crushes  it  between  its  thighs,  which  are  armed 
with  spines,  and  then  delivers  it  up  to  the  jaws  for 
mastication.  All  arthropods  move  their  jaws  horizon- 
tally. 

The  backboned  animals  generally  apprehend  food  by 
means  of  their  jaws,  of  which  there  are  two,  moving 
vertically.  The  toothless  sturgeon  draws  in  its  prey  by 
powerful  suction.  The  hagfish  has  a  single  tooth,  which 
it  plunges  into  the  sides  of  its  victim,  and,  thus  securing 


HOW  ANIMALS   EAT 


255 


a  firm  hold,  bores  its  way  into  the  flesh  by  means  of  its 
sawlike  tongue.  But  fishes  are  usually  well  provided 
with  teeth,  which,  being  sharp  and  curving  inward,  are 
strictly  prehensile.  The  fins  and  tongue  are  not  prehen- 
sile. A  mouth  with  horny  jaws,  as  in  the  turtles,  or 
bristling  with  teeth,  as  in  the  crocodile,  is  the  only 
means  possessed  by  nearly  all  amphibians  and  reptiles 
for  securing  food.  The  toad,  frog,  and  chameleon  cap- 
ture insects  by  darting  out  the  tongue,  which  is  tipped 
with  glutinous  saliva.  The  constricting  serpents  (boas) 
crush  their  prey  in  their  coils  before  swallowing ;  and 
the  venomous  snakes  have  poison  fangs.  No  reptile 
has  prehensile  lips.  All  birds  use  their  toothless  beaks 
in  procuring  food,  but  birds  of 
prey  also  seize  with  their  tal- 
ons, and  woodpeckers,  hum- 
mers, and  parrots  with  their 
tongues.  The  beak  varies 
greatly  in  shape,  being  a  hook 
in  the  eagle,  a  probe  in  the 
woodpecker,  and  a  shovel  in 
the  duck. 

Among  the  quadrupeds  we 
find  a  few  special  contrivances, 
as  the  trunk  of  the  elephant, 
and  the  long  tongues  of  the 
giraffe  and  ant-eater;  but,  as 
a  rule,  the  teeth  are  the  chief 
organs  of  prehension,  'always  FIG.  2i7.— A™  of  the 
aided  more  or  less  by  the  lips.  Monkey  (Ateles}' 

Ruminants,  like  the  ox,  having  hoofs  on  their  feet, 
and  no  upper  front  teeth,  employ  the  lips  and  tongue. 
Such  as  can  stand  erect  on  the  hind  legs,  as  the 
squirrel,  bear,  and  kangaroo,  use  the  front  limbs  for 
holding  the  food  and  bringing  it  to  the  mouth,  but 


256  COMPARATIVE   ZOOLOGY 

never  one  limb  alone.  The  clawed  animals,  like  the 
cat  and  lion,  make  use  of  their  feet  in  securing  prey,  all 
four  limbs  being  furnished  with  curved  retractile  claws ; 
but  the  food  is  conveyed  into  the  mouth  by  the  move- 
ment of  the  head  and  jaws.  Man  and  the  monkeys  em- 
ploy the  hand  in  bringing  food  to  the  mouth,  and  the 
lips  and  tongue  in  taking  it  into  the  cavity.  The  thumb 
on  the  human  hand  is  longer  and  more  perfect  than 
that  of  the  apes  and  monkeys ;  but  the  foot  of  the  latter 
is  also  prehensile. 

2.  The  Mouths  of  Animals.  —  In  the  parasites,  as  the 
tapeworm,  which  absorb  nourishment  through  the  skin, 
and  insects,  as  the  May  fly  and  botfly,  which  do  all  their 
eating  in  the  larval  state,  the  mouth  is  either  wanting  or 
rudimentary.  The  amoeba,  also,  has  no  mouth  proper, 
its  food  passing  through  the  firmer  outside  part  of  the 
bit  of  protoplasm  which  constitutes  its  body.  Mouth 
and  anus  are  thus  extemporized,  the  opening  closing  as 
soon  as  the  food  or  excrement  has  passed  through. 

In  the  infusoria  the  "  mouth  "  is  a  round  or  oval  open- 
ing leading  through  the  cuticle  and  outer  layer  of  proto- 
plasm to  the  interior  of  the  single  cell  which  makes 
their  body.  It  is  usually  bordered  with  cilia,  and  situ- 
ated on  the  side  or  at  one  end  of  the  animal  (Figs.  9,  1 1). 

An  ellipticah or  quadrangular  orifice,  surrounded  with 
tentacles,  and  leading  directly  to  the  stomach,  is  the 
ordinary  mouth  of  the  polyps  and  jellyfishes.  In  those 
which  are  fixed,  as  the  actinia,  coral,  and  hydra,  the 
mouth  looks  upward  or  downward,  according  to  the 
position  in  which  the  animal  is  attached  (Figs.  17,  34, 
236);  in  those  which  freely  move  about,  as  the  jellyfish, 
it  is  generally  underneath,  the  position  of  the  animal 
being  reversed  (Fig.  22).  In  some,  the  margin,  or  lip, 
is  protruded  like  a  proboscis;  and  in  all  it  is  exceed- 
ingly dilatable. 


HOW  ANIMALS    EAT  257 

The  mouth  of  the  starfish  and  sea  urchin  is  a  simple 
round  aperture,  followed  by  a  very  short  throat.  In  the 
starfish,  it  is  inclosed  by  a  ring  of  hard  spines  and  a 
membrane.  In  the  sea  urchin  it  is  surrounded  by  a 
muscular  membrane  and  minute  tentacles,  and  is  armed 
with  five  sharp  teeth,  set  in  as  many  jaws,  resembling 
little  conical  wedges  (Fig.  226). 

Among  the  headless  mollusks,  the  oral  apparatus  is 
very  simple,  being  inferior  to  that  of  some  of  the  radiate 
animals.  In  the  oyster  and  bivalves  generally,  the  mouth 
is  an  unarmed  slit  —  a  mere  inlet  to  the  esophagus,  situT 
ated  in  a  kind  of  hood  formed  by  the  union  of  the  gills 
at  their  origin,  and  between  two  pairs  of  delicate  flaps, 
or  palpi.  These  palpi  make  a  furrow,  along  which  pass 
the  particles  of  food  drawn  in  by  the  cilia,  borne  by 
cells  which  cover  the  surface  of  the  flaps. 

Of  the  higher  mollusks,  the  little  clio  (one  of  the 
pteropods)  has  a  triangular  mouth,  with  two  jaws  armed 
with  sharp  horny  teeth,  and  a  tongue  covered  with  spiny 
booklets  all  directed  backward.  Some  univalves  have  a 
simple  fleshy  tube  or  siphon. 
Others,  as  the  whelk,  have 
an  extensible  proboscis, 
which  unfolds  itself,  like  the  V  y  \ 
finder  of  a  glove,  and  carries  FIG.  218.- jaw  of  the 
within  it  a  rasplike  tongue,  (Helix  albolabr^  P~tiy  magnified. 
which  can  bore  into  the  hardest  shells.  Such  as  feed 
on  vegetable  matter,  as  the  snail,  have  no  proboscis,  but 
on  the  roof  of  the  mouth  a  curved  horny  plate  fitted  to 
cut  leaves,  etc.,  which  are  pressed  against  it  by  the  lips, 
and  on  the  floor  of  the  mouth  a  small  tongue  covered 
with  delicate  teeth.  As  fast  as  the  tongue  is  worn  off 
by  use,  it  grows  out  from  the  root. 

The  mouth  of  the  cuttlefish  is  the  most  elevated  type 
below  that  of  the  fishes.     A  broad  circular  lip  nearly 
DODGE'S  GEN.  ZOOL. —  17 


258  COMPARATIVE   ZOOLOGY 

conceals  a  pair  of  strong  horny  mandibles,  not  unlike 
the  beak  of  a  parrot,  but  reversed,  the  upper  mandible 
being  the  shorter  of  the  two,  and  the  jaws,  which  are 
cartilaginous,  are  imbedded  in  a  mass  of  muscles,  and 
move  vertically.  Between  them  is  a  fleshy  tongue 
covered  with  teeth. 

The  parasitic  worms,  living  within  or  on  the  outside 
of  other  animals,  generally  have  a  sucker  at  one  end  or 
underneath,  serving  simply  for  attachment,  and  another 
which  is  perforated.  The  latter  is  a  true  suctorial  mouth, 
being  the  sole  inlet  of  food.  It  is  often  surrounded  with 
booklets  or  teeth,  which  serve  both  to  scarify  the  victim 
and  secure  a  firm  hold.  In  the  leech,  the  mouth  is  a 
triangular  opening  with*  thick  lips,  the  upper  one  pro- 
longed, and  with  three  jaws.  In  many  worms  it  is  a 
fleshy  tube,  which  can  be  drawn  in  or  extended,  like  the 
eye  stalks  of  the  snail,  and  contains  a  dental  apparatus 
inside  (Fig.  215). 

Millepedes  and  centipedes  have  two  lateral  jaws  and  a 
four-lobed  lip. 

In  lobsters  and  crabs  the  mouth  is  situated  underneath 
the  head,  and  consists  of  a  soft  upper  lip,  then  a  pair  of 
upper  jaws  provided  with  a  short  feeler,  below  which  is  a 
thin  bifid  lower  lip  ;  then  follow  two  pairs  of  membranous 
under  jaws,  which  are  lobed  and  hairy ;  and  next,  three 
pairs  of  foot  jaws  (Fig.  54).  The  horseshoe  crab  has 
no  special  jaws,  the  thighs  answering  the  purpose.  The 
barnacle  has  a  prominent  mouth,  with  three  pairs  of 
rudimentary  jaws. 

With  few  exceptions,  the  mouths  of  insects  in  the  lar- 
val state  are  fitted  only  for  biting,  the  two  jaws  being 
horny  shears.  But  in  the  winged,  or  perfect,  state, 
insects  may  be  divided  into  the  masticating  (as  the 
beetle)  and  the  suctorial  (as  the  butterfly).  In  the  for- 
mer group,  the  oral  apparatus  consists  of  two  pairs  of 


HOW  ANIMALS    EAT 


259 


horny  jaws  (mandibles  and  inaxillce\  which  work  hori- 
zontally between  an  upper  (labrum}  and  an  under  (la- 
bium} lip.  The  maxillae  and  under  lip  carry  sensitive 
jointed  feelers  (palpi).  The  front  edge  of  the  labium  is 


FIG.  219.  —  Mouth  of  a  Locust,  dissected  and  much  magnified:  i,  labrum,  or  upper  lip; 
2,  mandibles;  3,  jaws;  4,  labium,  or  lower  lip;  5,  tongue.  The  appendages  to  the 
maxillae  and  lower  lip  are  palpi. 

commonly  known  as  the  tongue  (ligtdd)^  In  such  a 
mouth,  the  mandibles  are  the  most  important  parts  ; 
but  in  passing  to  the  suctorial  insects,  we  find  that  the 
mandibles  are  secondary  to  the  maxillae  and  labium, 
which  are  the  only  means  of  taking  food.  In  the  bee 


260 


COMPARATIVE   ZOOLOGY 


tribe,  we  have  a  transition  between  the  biting  and  the 
sucking  insects  —  the  mandibles  "supply  the  place  of 
trowels,  spades,  pickaxes,  saws,  scissors,  and  knives," 
while  the  maxillae  are  developed  into  a  sheath  to  inclose 
the  long,  slender,  hairy  tongue  which  laps  up  the  sweets 
of  flowers.  In  the  suctorial  butterfly,  the  lips,  mandi- 
bles, and  palpi  are  reduced  to  rudiments,  while  the 

maxillae  are  excessively 
lengthened  into  a  proboscis, 
their  edges  locking  by  means 


FIG.  220.  —  Head  of  a  Wild  Bee  (An- 
thophora  retusa),  magnified,  front 
view:  a,  compound  eyes;  b, 
clypeus;  c,  three  simple  eyes;  d, 
antennae;  <?,  labrum;  f,  mandibles; 
z,  maxillae;  h,  maxillary  palpi;  /, 
palpifer;  /,  labial  palpi;  m,  para- 
glossae;  k,  ligula. 


FIG.  221.  —  Proboscis  of  a  Butterfly,  magnified. 


of  minute  teeth,  so  as  to  form 
a  central  canal,  through  which 
the  liquid  food  is  pumped  up 
into  the  mouth.  Seen  under 
the  microscope,  the  proboscis  is  made  up  of  innumerable 
rings  interlaced  with  spiral  muscular  fibers.  The  probos- 
cis of  the  fly  is  a  modified  lower  lip  ;  that  of  the  bugs  and 
mosquitoes,  fitted  both  for  piercing  and  suction,  is  formed 
by  the  union  of  four  bristles,  which  are  the  mandibles 
and  maxillae  strangely  altered,  and  encased  in  the  labium 
when  not  in  use. 


HOW  ANIMALS   EAT 


26l 


As  most  of  the  arachnids  live  by  suction,  the  jaws 
are  seldom  used  for  mastication.  In  the  scorpion,  the 
apparent  representatives  of  the  mandibles  of  an  insect 
are  transformed  into  a  pair  of  small  forceps,  and  the 
palpi,  so  small  in  insects,  are  developed  into  formidable 
claws :  both  of  these  or- 
gans are  prehensile.  In 
spiders,  the  so-called  man- 
dibles, which  move  more 
or  less  vertically,  end  in 


FIG.  222.  —  Mouth  of  the  Horsefly  ( Taba- 
nus  lineola),  magnified:  a,  antennae; 
nt,  mandibles;  tnx,  maxillae;  mp, 
maxillary  palpi ;  Ib,  labrum;  /,  labium, 
or  "  tongue." 

a  fang;  and  the  clublike 
palpi,  often  resembling 
legs,  have  nothing  to  do 
with  ingestion  or  locomo- 
tion. Both  scorpions  and 

spiders  have  a  soft  upper   FlG-  223' -Under  Surface  of  Male  sPider' 
lip,  and  a  groove  within 
the  mouth,  which  serves 
as  a  canal  while  sucking 
their  prey.     The  tongue  is  external,  and  situated  be- 
tween a  pair  of  diminutive  maxillae. 

In  the  ascidians  the  first  part  of  the  alimentary  canal 
is  enormously  enlarged  and  modified  to  serve  as  a  gill 
sac.  At  the  bottom  of  this  sac,  and  far  removed  from 


enlarged:  a,  c,  poison  fang;  b,  teeth  on  in- 
terior margin  of  mandible,  e  ;  f,  labium ;  g, 
thorax;  h,  limbs;  i,  abdomen;  /.spinnerets; 
m,  maxillary  palpus;  d,  dilated  terminal 
joint. 


262  COMPARATIVE   ZOOLOGY 

its  external  opening,  lies  the  entrance  to  the  digestive 
tract  proper.  Into  it  the  particles  of  food  entering  with 
the  water  are  conveyed  (Fig.  115). 

The  mouth  of  vertebrates  is  a  cavity  with  a  fixed  roof 
(the  hard  palate)  and  a  movable  floor  (the  tongue  and 
lower  jaw),  having  a  transverse  opening  in  front,76  and 
a  narrow  outlet  behind,  leading  to  the  gullet.  Save  in 
birds  and  some  others,  the  cavity  is  closed  in  front  with 
lips,  and  the  margins  of  the  jaws  are  set  with  teeth. 

In  fishes  the  mouth  is  the  common  entry  to  both  the 
digestive  and  respiratory  organs ;  it  is,  therefore,  large, 
and  complicated  by  a  mechanism  for  regulating  the 
transit  of  the  food  to  the  stomach  and  the  aerated  water 
to  the  gills.  The  slits  leading  to  the  gills  are  provided 
with  rows  of  processes  which,  like  a  sieve,  prevent  the 
entrance  of  food,  and  with  valves  to  keep  the  water, 
after  it  has  entered  the  gills,  from  returning  to  the 
mouth.  So  that  the  mouths  of  fishes  may  be  said  to 
be  armed  at  both  ends  with  teeth-bearing  jaws.  A  few 
fishes,  as  the  sturgeon,  are  toothless ;  but,  as  a  class, 
they  have  an  extraordinary  dental  apparatus  —  not  only 
the  upper  and  lower  jaws,  but  even  the  palate,  tongue, 
and  throat  being  sometimes  studded  with  teeth.  Every 
part  of  the  mouth  is  evidently  designed  for  prehension 
and  mastication.  Lips  are  usually  present;  but  the 
tongue  is  often  absent,  or  very  small,  and  as  often  aids 
respiration  as  ingestion. 

Amphibians  and  reptiles  have  a  wide  mouth ;  even 
the  insect-feeding  toads  and  the  serpents  can  stretch 
theirs  enormously.  True  fleshy  lips  are  wanting  ;  hence 
the  savage  aspect  of  the  grinning  crocodile.  With  some 
exceptions,  as  toads  and  turtles,  the  jaws  are  armed 
with  teeth.  Turtles  are  provided  with  horny  beaks. 
The  tongue  is  rarely  absent,  but  is  generally  too  thick 
and  short  to  be  of  much  use.  In  the  toad  and  frog  it 


HOW  ANIMALS    EAT  263 

is  singularly  extensile ;  rooted  in  front  and  free  behind, 
it  is  shot  from  the  mouth  with  such  rapidity  that  the 
insect  is  seized  and  swallowed  more  quickly  than  the 
eye  can  follow.  The  chameleon's  tongue  is  also  exten- 
sile. Snakes  have  a  slender  forked  tongue,  consisting 
of  a  pair  of  muscular  cylinders,  which  is  solely  an  in- 
strument of  touch. 


FlG.  224. — Mouth  of  the  Crocodile:  d,  tongue;  e,  glands;  _/",  inferior,  and  g,  superior, 
valve,  separating  the  cavity  of  the  mouth  from  the  throat,  h. 

Birds  are  without  lips  or  teeth,  the  jaws  being  cov- 
ered with  horn  forming  a  beak.  This  varies  greatly 
in  shape,  being  extremely  wide  in  the  whip-poor-will, 
remarkably  long  in  the  pelican,  stout  in  the  eagle,  and 
slender  in  the  hummer.  It  is  hardest  in  those  that  tear 
or  bruise  their  food,  and  softest  in  water  birds.  The 
tongue  is  also  covered  with  a  horny  sheath,  and  is  gen- 
erally spinous,  its  chief  function  being  to  secure  the 
food  when  in  the  mouth.  It  is  proportionally  largest 
and  most  fleshy  in  the  parrots. 

The  main  characteristics  of  the  mammalian  mouth 
are  flesh  lips  and  mobile  cheeks.77  In  the  duck-billed 


264 


COMPARATIVE   ZOOLOGY 


monotremes  lips  are  wanting,  and  in  the  porpoises  they 
are  barely  represented.  But  in  the  herbivorous  quadru- 
peds they,  with  the  tongue,  are  the  chief  organs  of  pre- 
hension ;  in  the  carnivorous  tribes  they  are  thin  and 
retractile  ;  while  in  the  whale  the  upper  lip  falls  down 
like  a  curtain,  overlapping  the  lower  jaw  several  feet. 
As  a  rule,  the  mouth  is  terminal ;  but  in  the  elephant, 
tapir,  hog,  and  shrew,  the  upper 
lip  blends  with  the  nose  to 
form  a  proboscis,  or  snout.  The 
mouth  is  comparatively  small  in 
the  elephant  and  in  gnawing 
animals  like  the  squirrel,  wide 
in  the  carnivores,  short  in  the 
sloth,  and  long  in  the  ant-eater. 
Teeth  are  usually  present,  but 
vary  in  form  and  number  with 
the  habits  of  the  animal.  The 
ant-eater  is  toothless,  and  the 
Greenland  whale  has  a  sieve 

FIG.  225.  —  Human  Tongue  and  ad-    made     of      homy      platCS.          The 
jacent  parts:    a,  lingual  papillae; 

b,   papiiise    forming    V-shaped  tongue    conforms    in    size    and 

lines;    d,   fungiform    papillae;     e,       ,  .,••      ,-,         -,  .  j 

filiform  papillae;   g,  epiglottis;  shape  with  the  lower  jaw,   and 

-.,    uvula      or    conical     process,  '^    a     muscular      sensitive    Organ, 

hanging     from     the     soft     palate, 

n;    o,    hard    palate;     r,    palatine  which       SCrVCS      many 

glands,    the     mucous     membrane  . 

being  removed;    v,  section  of  the  assisting  in  the  prehension, 

tication,  arid  swallowing  of  food, 

besides  being  an  organ  of  taste,  touch,  and  speech.  Its 
surface  is  covered  with  minute  prominences,  called 
papilla,  which  are  arranged  in  lines  with  mathematical 
precision.  In  the  cats,  these  are  developed  into  recurved 
spines,  which  the  animal  uses  in  cleaning  bones  and 
combing  its  fur.  Similar  papillae  occur  on  the  roof  and 
sides  of  the  mouth  of  the  ox  and  other  ruminants.  In 
some  animals,  as  the  hamster  and  gopher,  the  cheeks 


HOW  ANIMALS   EAT 


265 


are  developed  into  pouches  in  which  the  food  may  be 
carried.  These  may  be  lined  with  hair.  The  tongue  is 
remarkably  long  in  the  ant-eater  and  giraffe,  and  almost 
immovable  in  the  gnawers,  elephants,  and  whales. 

3.  The  Teeth  of  Animals.  —  Nearly  all  animals  have 
certain  hard  parts  within  the  mouth  for  the  prehension 
or  trituration  of 
solid  food.  If 
these  are  want- 
ing, the  legs  are 
of  ten  armed  with 
spines,  or  pin- 
cers, to  serve 
the  same  pur- 
pose, as  in  the 
horseshoe  crab; 
or  the  stomach 
is  lined  with 
"gastric  teeth,"  as  in  some  marine  snails;  or  the  defi- 
ciency is  supplied  by  a  muscular  gizzard,  as  in  birds, 

ant  -  eaters,  and 
some  insects. 
Even  the  lobster 
and  crab,  in  ad- 
dition  to  their 
complicated  oral 


FIG.  226.  — Sea  Urchin  bisected,  showing  masticating 
apparatus. 


B 


FIG.  227.  -Teeth  and  Masticatory  Apparatus  of  Gastro-  Organs,     haVC  the 

pods,  enlarged:  A,  portion  of  odontophore,  or  "  tongue,"  stomach          fUT- 
of  Velutina,  enlarged;    B,   portion   of  odontophore  of 

Whelk  (Buccinum   undatunt),   magnified  —  the   entire  nished         With  3. 

tongue  has  too  rows  of  teeth;  C,  head  and  odontophore                       r     i'  f 

ofUmpettPattttavufeata);  D,  portion  of  same,  greatly  pOWeriUl.     SCt  OI 
magnified,  to  show  the  transverse  rows  of  siliceous  teeth. 


The  sea  urchin  is  one  of  the  lowest  animals  which 
exibits  anything  like  a  dental  apparatus.  Five  calcare- 
ous teeth,  having  a  wedge-shaped  apex,  each  set  in  a 
triangular  pyramid,  or  "jaw,"  are  moved  upon  each 


266 


COMPARATIVE   ZOOLOGY 


other  by  a  complex  arrangement  of  levers  and  muscles. 
Instead  of  moving  up  and  down,  as  in  vertebrates,  or 
from  right  to  left,  as  in  arthropods,  they  converge 
toward  the  center,  and  the  food  passes  between  ten 
grinding  surfaces. 

The  rotifers  have  a  curious  pair  of  horny  jaws.     That 
which  answers  to  the  lower  jaw  is  fixed,  and  called  the 

"  anvil."  The  upper  jaw 
consists  of  two  pieces  called 
"hammers,"  which  are 
sharply  notched,  and  beat 
upon  the  "  anvil  "  between 
them  (Fig.  40). 

The  horny-toothed  mandi- 
bles of  insects,  already  men- 
tioned, are  prehensile,  and 
also  serve  to  divide  the  food. 
The  three  little  white 
ridges  in  the  mouth  of  the 
leech  are  the  convex  edges 
of  horny  semicircles,  each 
bordered  by  a  row  of  nearly 
a  hundred  hard,  sharp  teeth. 
When  the  mouth,  or  sucker, 
is  applied  to  the  skin,  a  saw- 
ing movement  is  given  to 
the  horny  ridges,  so  that 
the  "  bite "  of  the  leech  is 
really  a  saw  cut. 

The  dentition  of  the  uni- 
valve mollusks,  or  the  snails, 
is  generally  lingual,  i.-e.,  it 
consists  of  microscopic  teeth,  usually  siliceous  and 
amber-colored,  planted  in  rows  on  the  tongue.  The 
teeth  are,  in  fact,  the  serrated  edges  of  minute  plates. 


FIG.  228.  —  Section  of  one  half  of  the  Up- 
per Jaw  of  a  Whale  (Balasnoptera), 
showing  baleen  plates:  a,  superior 
maxillary  bone;  6,  ligamentous  gum 
attaching  the  horny  body  of  the  baleen 
plate,  c;  d,  fringe  of  bristles ;  e,  smaller 
plates. 


HOW  ANIMALS   EAT 


267 


The  number  of  these  plates  varies  greatly  ;  the  garden 
slug  has  1  60  rows,  with  180  teeth  in  each  row. 

All  living  birds,  and  some  other  vertebrates,  as  ant- 
eaters,78  turtles,  tortoises,  toads,  and  sturgeons,  are  with- 
out teeth.  Their  place  is  often  supplied  by  a  horny 
beak,  a  muscular  gizzard,  or  both  structures. 

In  a  few  vertebrates,  horny  plates  take  the  place  of 
teeth,  as  the  duck  mole  (Ornithorhynchus)  and  whalebone 
whale.  In  the  former,  the  plates  consist  of  closely  set 
vertical  hollow  tubes  ;  in  the  latter,  the  baleen,  or  whale- 
bone, plates,  triangular  in  shape,  and  fringed  on  the 
inner  side,  hang  in  rows  from  the  gums  of  the  upper 
jaw.  In  some  whales  there  are  about  300  plates  on  each 
side.79 

True  teeth,  consisting  mainly  of  a  hard,  calcareous 
substance  called  dentine,  are  found  only  in  backboned 
animals.  They  are  distinct  from 
the  skeleton,  and  differ  from 
bone  in  containing  more  mineral 
matter,  and  in  not  showing,  under 
the  microscope,  any  minute  cav- 
ities, called  lacuna.  A  typical 
tooth,  as  found  in  man,  consists 
of  a  central  mass  of  dentine, 
capped  with  enamel,  and  sur- 
rounded on  the  fang  with  cement. 
The  first  tissue  is  always  pres- 
ent, while  the  others  may  be 

T  T  ,    •  •    ,  f  • 

absent.     It  is  a  mixture  of  am- 

mal  and  mineral  matter  disposed 

in  the  form  of   extremely  fine 

tubes  and  cells,  so  minute  as  to  prevent  the  admission 

of  the  red  particles  of  blood.     One  modification  of  it  is 

ivory,  seen  in  'the  tusks  of  elephants.     Enamel  is  the 

hardest  tissue  of  the  body,  and  contains  not  more  than 


FIG.  229.  —  Section  of  Human  MO- 

lar,    enlarged:     k,    crown;      n, 

nec'k.  /(  ££.  ,;  enamel.  ^ 
dentine;   c>  cement:  >»  PUIP 

cavity. 


268 


COMPARATIVE   ZOOLOGY 


two  per  cent  of  animal  matter.  It  consists  of  six-sided 
fibers  set  side  by  side,  at  right  angles  to  the  surfaces 
of  the  dentine.  Cement  closely  resembles  bone,  and  is 
present  in  the  teeth  of  only  the  higher  animals. 

Teeth  are  usually  confined  to  the  jaws  ;  but  the  num- 
ber, size,  form,  structure,  position,  and  mode  of  attach- 
ment vary  with  the  food  and  habits  of  the  animal.  As 
a  rule,  animals  developing  large  numbers  of  teeth  in  the 
back  part  of  the  mouth  are  inferior  to  those  having 
fewer  teeth,  and  those  nearer  the  lips.  The  teeth  of 
only  mammals  have  fangs. 

The  teeth  of  fishes  present  the  greatest  variety.  In 
number,  they  range  from  zero  to  hundreds.  The  hag- 
fish  {Myxine)  has  a  single 
tooth  on  the  roof  of  the 
mouth,  and  two  serrated 
plates  on  the  tongue;  while 
the  mouth  of  the  pike  is 
crowded  with  teeth.  In 
some  we  find  teeth  short  and 
blunt,  in  the  shape  of  cubes, 


FIG.  230.— jaws  and  Pavement  teeth  of  a   or  prisms,  arranged  like  mo- 

saic  work.     Such  pavement 

teeth  (seen  in  some  rays)  are  fitted  for  grinding  seaweed 
and  crushing  shellfish.  But  the  cone  is  the  most  common 
form  :  sometimes  so  slender  and  close  as  to  resemble 
plush,  as  in  the  perch  ;  or  of  large  size,  and  flattened  like 
a  spearhead  with  serrated  edges,  as  in  the  shark ;  but 
more  often  like  the  canines  of  mammals,  curved  inward 
to  fit  them  for  grappling.  In  the  shark,  the  teeth  are  con- 
fined to  the  fore  part  of  the  mouth ;  in  the  carp,  they 
are  all  situated  on  the  bones  of  the  throat ;  in  the  parrot 
fish,  they  occupy  both  back  and  front ;  but  in  most  fishes 
the  teeth  are  developed  also  on  the  roof,'  or  palate,  and, 
in  fact,  on  nearly  every  bone  in  the  mouth.  They  seldom 


.     HOW  ANIMALS    EAT  269 

appear  (as  in  the  salmon)  on  the  upper  maxillary.  As 
to  mode  of  attachment,  the  teeth  are  generally  anchy- 
losed  (fastened  by  bony  matter)  to  the  bones  which  sup- 
port them,  or  simply  bound  by  ligaments,  as  in  the  shark. 
In  a  few  fishes,  the  teeth  consist  of  flexible  cartilage ; 
but  almost  invariably  they  are  composed  of  some  kind 
of  dentine,  enamel  and  cement  being  absent. 

Of  amphibians  and  reptiles,  toads,  turtles,  and  tor- 
toises are  toothless ;  frogs  have  teeth  in  the  upper  jaw 
only;  snakes  have  a  more 
complete  set;  but  saurians 
possess  the  most  perfect 
dentition.  The  number  is 
not  fixed  even  in  the  same 
species;  in  the  alligator  it 
varies  from  72  to  88.  The 

teeth     are      limited      tO     the      FIG.  231. -Poison  Apparatus  of  the  Rattle- 

snake:  g,  gland,  with  duct,  leading  to  the 

jawbones  in  the  higher  fang,//  *«,  elevator  muscles  of  the  jaw, 
r  ,  •  \  i  •  which,  in  contracting,  compress  the  gland; 

lOrniS     (Saurians);       but      in         s,  salivary  glands  on  the  edge  of  the  jaws; 

others,  as  the  serpents,  w>nostril- 
they  are  planted  also  in  the  roof  of  the  mouth.  With 
few  exceptions,  they  are  conical  and  curved  (Fig.  224). 
In  the  serpents  they  are  longest  and  sharpest;  and 
the  venomous  species  have  two  or  more  fangs  in  the 
upper  jaw.  These  fangs  contain  a  canal,  through  which 
the  poison  is  forced  by  muscles  which  compress  the 
gland.  The  bones  to  which  they  are  attached  are  mov- 
able, and  the  fangs  ordinarily  lie  flat  upon  the  gums,  but 
are  brought  into  a  vertical  position  in  the  act  of  striking. 
As  a  rule,  the  teeth  of  reptiles  are  simply  soldered  to 
the  bone  which  supports  them,  or  lodged  in  a  groove ; 
but  those  of  crocodiles  are  set  in  sockets.  Reptilian 
teeth  are  made  of  dentine  and  a  thin  layer  of  cement, 
to  which  is  added  in  most  saurians  a  coat  of  enamel  on 
the  crown. 


2/0  COMPARATIVE   ZOOLOGY, 

In  the  majority  of  mammals,  the  teeth  are  limited  in 
number  and  definite  in  their  forms.  The  number  ranges 
from  i  in  the  narwhal  (but  the  longest  tooth  in  the 
animal  kingdom)  to  220  in  the  dolphin.  The  average  is 
32,  occurring  in  ruminants,  apes,  and  man ;  but  44  (as 
in  the  hog  and  mole)  is  called  the  typical  or  normal 
nutrfber,  and  this  number  is  exceeded  only  in  the  lower 


FIG.  232.  —  Skull  of  the  Babirusa,  or  Malayan  Hog,  showing  growth  and  curvature  of  the 

canines. 

groups.  When  very  numerous,  the  teeth  are  of  the 
reptilian  type,  small,  pointed,  and  of  nearly  equal  size, 
as  in  the  porpoise.  In  the  higher  mammals,  the  teeth 
are  comparatively  few,  and  differ  so  much  in  size,  shape, 
and  use,  that  they  can  be  classed  into  incisors,  canines, 
premolars,  and  molars.  Such  a  dental  series  exhibits 
a  double  purpose,  prehension  and  mastication.  The 


HOW  ANIMALS    EAT  2/1 

chisel-shaped  front  teeth  are  fitted  for  cutting  the  food, 
and  hence  called  incisors.  These  vary  in  number :  the 
lion  has  six  in  each  jaw ;  the  squirrel  has  two  in  each 
jaw,  but  remarkably  developed ;  the  ox  has  none  in  the 
upper  jaw,  and  the  elephant  none  in  the  lower ;  while 
the  sloth  has  none  at  all.80  The  canines,  so  called 
because  so  prominent  in  the  dog,  are  conical,  and, 
except  in  man,  longer  than  the  other  teeth.  They  are 
designed  for  seizing  and  tearing ;  and  they  are  the  most 
formidable  weapons  of  the  wild  carnivores.  There  are 
never  more  than  four.  They  are  wanting  in  all  rodents, 
and  in  nearly  all  herbivorous  quadrupeds.  The  molars, 
or  grinders,  vary  greatly  in  shape,  but  closely  correspond 
with  the  structure  and  habits  of  the  animal,  so  that  a 
single  tooth  is  sufficient  to  indicate  the  mode  of  life 
and  sometimes  to  identify  the  species.81  In  the  rumi- 
nants, rodents,  horses,  and  elephants,  the  summits  of 
the  molars  are  flat,  like  millstones,  with  transverse  or 
curving  ridges  of  enamel.  In  the  cats  and  dogs,  they 
are  narrow  and  sharp,  passing  by  each  other  like  the 
blades  of  scissors,  and  therefore  cutting,  rather  than 
grinding,  the  food.  The  more  purely  carnivorous  the 
species,  and  the  more  it  feeds  upon  living  prey,  the  fewer 
the  molars.  In  animals  living  on  mixed  diet,  as  the  hog 
and  man,  the  crowns  have  blunt  tubercles.  Premolars, 
or  bicuspids,  are  those  which  were  preceded  by  milk 
teeth ;  the  true,  or  back,  molars  had  no  predecessors. 

The  dentition  of  mammals  is  expressed  by  a  formula, 
which  is  a  combination  of  initial  letters  and  figures  in 
fractional  form,  to  show  the  number  and  kind  of  teeth 
on  each  side  of  both  jaws.  Thus,  the  formula  for  man 


The  teeth  of  mammals  are  always  restricted  to  the 
margins  of  the  jaws,  and  form  a  single  row  in  each. 


2/2  COMPARATIVE   ZOOLOGY 

But  they  rarely  form  an  unbroken  series.82  The  teeth 
implanted  in  the  premaxillary  bone,  and  in  the  corre- 
sponding part  of  the  lower  jaw,  whatever  their  number, 
are  incisors.  The  first  tooth  bjehind  the  premaxillary,  if 
sharp  and  projecting,  is  a  canine. 


nts 


FIG.  233.  —  Teeth  of  the  right  lower  jaw  of  adult  male  Chimpanzee  (Anthropopithecus 
troglodytes),  natural  size.     The  molar  series  does  not  form  a  curve,  as  in  Man. 

Each  tooth  has  its  particular  bony  socket.83  The 
molars  may  be  still  further  strengthened  by  having  two 
or  more  diverging  fangs,  or  roots,  a  feature  peculiar  to 
this  class.  The  incisors  and  canines  have  but  one  fang  ; 
and  those  that  are  perpetually  growing,  as  the  incisors 
of  rodents  and  elephants,  have  none  at  all.  The  teeth 
of  flesh-eating  mammals  usually  consist  of  hard  dentine, 
surrounded  on  the  root  with  cement  and  capped  with 
enamel.  In  the  herbivorous  tribes,  they  are  very  com- 
plex, the  enamel  and  cement  being  inflected  into  the 
dentine,  forming  folds,  as  in  the  molar  of  the  ox,  or 
plates,  as  in  the  compound  tooth  of  the  elephant.  This 
arrangement  of  these  tissues,  which  differ  in  hardness, 
secures  a  surface  with  prominent  ridges,  well  adapted 
for  grinding.  The  cutting  teeth  of  the  rodents  consist 
of  dentine,  with  a  plate  of  enamel  on  the  anterior  sur- 
face, and  the  unequal  wear  preserves  a  chisel-like  edge. 


HOW  ANIMALS    EAT  273 

Enamel  is  sometimes  wanting,  as  in  the  molars  of  the 
sloth  and  the  tusks  of  the  elephant. 

In  fishes  and  reptiles,  there  is  an  almost  unlimited 
succession  of  teeth ;  but  mammalian  teeth  are  cast  and 
renewed  but  once  in  life. 

Vertebrates  use  their  teeth  for  the  prehension  of  food, 
as  weapons  of  offense  or  defense,  as  aids  in  locomotion, 
and  as  instruments  for  uprooting  or  cutting  down  trees. 
But  in  the  higher  class  they  are  principally  adapted  for 
dividing  or  grinding  the  food.84  While  in  nearly  all 


FlG.  234.  —  Upper  Molar  Tooth  of  Indian  Elephant  (Elephas  indicus},  showing  trans- 
verse arrangement  of  dentine,  d,  with  festooned  border  of  enamel  plates,  e;  c, 
cement;  one-third  natural  size. 

other  vertebrates  the  food  is  bolted  entire,  mammals 
masticate  it  before  swallowing.  Mastication  is  more 
essential  in  the  digestion  of  vegetable  than  of  animal 
food ;  and  hence  we  find  the  dental  apparatus  most  effi- 
cient in  the  herbivorous  quadrupeds.  The  food  is  most 
perfectly  reduced  by  the  rodents. 

Teeth,  as  we  shall  see,  are  appendages  of  the  skin, 
not  of  the  skeleton,  and,  like  other  superficial  organs, 
are  especially  liable  to  be  modified  in  accordance  with 
the  habits  of  the  creature.  They  are,  therefore,  of  great 
zoological  value  ;  for  such  is  the  harmony  between  them 
and  their  uses,  the  naturalist  can  predict  the  food  and 
general  structure  of  an  animal  from  a  sight  of  the  teeth 
alone.  For  the  same  reason,  they  form  important 
DODGE'S  GEN.  ZOOL. —  18 


2/4  COMPARATIVE   ZOOLOGY 

guides  in  the  classification  of  animals ;  while  their 
durability  renders  them  available  to  the  paleontologist 
in  the  determination  of  the  nature  and  affinities  of  ex- 
tinct species,  of  which  they  are  often  the  sole  remains. 
Even  the  structure  is  so  peculiar  that  a  fragment  will 
sometimes  suffice. 

4.  Deglutition,  or  how  Animals  Swallow.  —  In  the 
lowest  forms  of  life,  the  mouth  is  but  an  aperture  open- 
ing immediately  into  the  body  substance,  and  the  food  is 
drawn  in  by  ciliary  currents  (Figs.  9,  n).  But  in  the 
majority  of  animals,  a  muscular  tube,  called  the  gullet,  or 
esophagus,  intervenes  between  the  mouth  and  stomach, 
the  circular  fibers  of  which  contract,  in  a  wavelike  man- 
ner, from  above  downward,  propelling  the  morsel  into  the 
stomach.85  In  the  higher  mollusks,  arthropods,  and  ver- 
tebrates, deglutition  is  generally  assisted  by  the  tongue, 
which  presses  the  food  backward,  and  by  a  glairy  juice, 
called  saliva,  which  facilitates  its  passage  through  the 
gullet.86  Vertebrates  have  a  cavity  behind  the  mouth, 
called  the  throat,  or  pharynx,  which  may  be  considered 
as  a  funnel  to  the  esophagus.87  In  air  breathers,  it  has 
openings  leading  to  the  windpipe,  nose,  and  ears.  In 
man,  as  in  mammals  generally,  the  process  of  deglutition 
is  in  this  wise :  the  food,  masticated  by  the  teeth  and 
lubricated  by  the  saliva,  is  forced  by  the  tongue  and 
cheeks  into  the  pharynx,  the  soft  palate  keeping  it  out 
of  the  nasal  aperture,  and  the  valvelike  epiglottis  falling 
down  to  form  a  bridge  over  the'  opening  to  the  wind- 
pipe. The  moment  the  pharynx  receives  the  food,  it  is 
firmly  grasped,  and,  the  muscular  fibers  contracting 
above  it  and  left  lax  below  it,  it  is  rapidly  thrust  into 
the  esophagus.  Here,  a  similar  movement  (the  peristal- 
tic) strips  the  food  into  the  stomach.88  The  rapidity  of 
these  contractions  transmitted  along  the  esophagus  may 
be  observed  in  the  neck  of  a  horse  while  drinking. 


HOW  ANIMALS   EAT  275 

Deglutition  in  the  serpents  is  painfully  slow,  and 
somewhat  peculiar.  For  how  is  an  animal,  without 
limbs  or  molars,  to  swallow  its  prey,  which  is  often 
much  larger  than  its  own  body  ?  The  boa  constrictor, 
e.g.,  seizes  the  head  of  its  victim  with  its  sharp,  recurv- 
ing teeth,  and  crushes  the, body  with  its  overlapping 
coils.  Then,  slowly  uncoiling,  and  covering  the  carcass 


FIG.  235.  —  Skull  of  Boa  Constrictor:  i,  frontal;  2,  prefrontal;  4,  postfrontal;  5,  basi- 
occipital;  6,  sphenoid;  7,  parietal;  12,  squamosal;  13,  prob'tic;  17,  premaxillary; 
18,  maxillary;  20,  nasal;  24,  transverse;  25,  internal  pterygoid;  34,  dentary,  lower 
jaw;  35,  angular;  36,  articular;  a,  quadrate;  s,  prenasal;  v,  petrosal. 

with  a  slimy  mucus,  it  thrusts  the  head  into  its  mouth 
by  main  force,  the  mouth  stretching  marvelously,  the 
skull  being  loosely  put  together.  One  jaw  is  then  un- 
fixed, and  the  teeth  withdrawn  by  being  pushed  forward, 
when  they  are  again  fastened  farther  back  upon  the 
animal.  The  other  jaw  is  then  protruded  and  refas- 
tened ;  and  thus,  by  successive  movements,  the  prey  is 
slowly  and  spirally  drawn  into  the  wide  gullet. 


CHAPTER   XII 

THE   ALIMENTARY   CANAL 

The  Alimentary  Canal  is  the  great  route  by  which 
nutritive  matter  reaches  the  interior  of  the  body.  It  is 
the  most  universal  organ  in  the  animal  kingdom,  and 
the  rest  are  secondary  or  subservient  to  it.  In  the 
higher  animals,  it  consists  of  a  mouth,  pharynx,  gullet, 
stomach,  and  intestine. 

It  is  a  general  law,  that  food  can  be  introduced  into 
the  living  system  only  in  a  fluid  state.  While  plants 
send  forth  their  roots  to  seek  nourishment  from  without, 
animals,  which  may  be  likened  to  plants  turned  outside 
in,  have  their  roots  (called  absorbents)  directed  inward 
along  the  walls  of  a  central  tube  or  cavity.  This  cavity 
is  for  the  reception  and  preparation  of  the  food,  so  that 
animals  may  be  said  to  carry  their  soil  about  with  them. 
The  necessity  for  such  a  cavity  arises  not  only  from  the 
fact  that  the  food,  which  is  usually  solid,  must  be  dis- 
solved, so  as  to  make  its  way  through  the  delicate  walls 
of  the  cavity  into  the  system,  but  also  from  the  occur- 
rence of  intervals  between  the  periods  of  eating,  and 
the  consequent  need  of  a  reservoir.  For  animals,  unlike 
plants,  are  thrown  upon  their  own  wits  to  procure  food. 

The  Protozoa,  as  the  amoeba  and  Infusoria,  can  not  be 
said  to  have  a  digestive  canal.  The  animal  is  here 
composed  of  a  single  cell,  in  which  the  food  is  digested. 
The  jelly  like  amoeba  passes  the  food  through  the  firmer 
outer  layer  (ectosarc)  into  the  more  fluid  inner  part 
(endosarc),  where  it  is  digested  (Fig.  i).  The  Infusoria, 

276 


THE   ALIMENTARY   CANAL 


277 


which  have  a  cuticle,  and  so  a  more  definite  form,  pos- 
sess a  mouth,  or  opening,  into  the  interior  of  their  cell 
body,  and  at  least  a  definite  place  where  the  excrement 
is  passed  out  (Figs,  9,  u).  But  we  can  not  call  this 
cell  cavity  a  digestive  tract. 

In  the  higher  animals,  the  alimentary  canal  is  a  con- 
tinuation of  the  skin,  which  is  reflected  inward,  as  we 
turn  the  finger  of  a  glove.89  We  find  every  grade  of 
this  reflection,  from  the  sac  of  the  hydra  to  the  long  in- 
testinal tube  of  the  ox.  So  that  food  in  the  stomach  is 
still  outside  of  the  true  body. 

The  simplest  form  of  such  a  digestive  tract  is  seen  in 
the  hydra  (Fig.  18).  Here  the  body  is  a  simple  bag, 
whose  walls  are 
composed  of  two 
layers  of  cells  (ecto- 
derm and  endo- 
denri).  A  mouth 
leads  into  the  cav- 
ity, and  serves  as 
well  for  the  out- 
let of  matter  not 
wanted.  The  en- 
dodermal  cells  fur- 
nish the  juices  by 
which  the  food  is 
digested  and  ab- 
sorb the  nutritious 
portions  of  it.  The 
polyps  have  also 
but  one  external  opening ;  but  from  this  hangs  down  a 
short  tube,  open  at  both  ends,  reaching  about  halfway  to 
the  bottom  of  the  body  cavity.  Such  an  arrangement 
would  be  represented  by  a  bottle  with  its  neck  turned 
inward.  In  this  suspended  sac,  which  is  somewhat  con- 


FIG.  236.  —Dissected  Actinia:  a,  the  thick  opaque  skin 
consisting  of  ectoderm,  lined  with  muscular  fibers; 
c ,  the  tubular  tentacles  communicating  with  the  inter- 
spaces, h,  between  the  membranous  vertical  folds; 
gi  S ' >  orifices  in  the  walls  allowing  passage  of  respira- 
tory water  from  one  compartment  to  another ;  d,  mouth 
leading  to  gastric  cavity,  e. 


2/8  COMPARATIVE   ZOOLOGY 

stricted  at  the  extremities,  digestion  takes  place ;  but  the 
product  passes  freely  into  all  the  surrounding  chambers, 
along  with  the  water  for  respiration  (Fig.  236).  The 
Medusae,  or  jelly  fishes,  preserve  the  same  type  of  a  diges- 
tive apparatus  ;  but  the  sac  is  cut  off  from  the  general 
cavity,  and  numerous  canals  radiate  from  it  to  a  circular 
canal  near  the  margin  of  the  disk  (Fig.  21).  In  the  star- 
fishes (Fig.  323),  we  find  a  great  advance.  The  saclike 
stomach  sends  off  two  glandular  branches  to  each  arm, 
which  doubtless  furnish  a  fluid  to  aid  in  digestion  (so- 
called  hepatic  coeca).  There  is  also  an  anus  present  in 
some  forms,  but  it  hardly  serves  to  pass  off  the  waste 
matter. 

Thus  far  we  have  seen  but  one  opening  to  the  diges- 
tive cavity,  rejected  portions  returning  by  the  same 
road  by  which  they  enter.  But  a  true  alimentary  canal 
should  have  an  anal  aperture  distinct  from  the  oral. 
The  simplest  form  of  such  a  canal  is  exhibited  by  the 
sponge,  in  its  system  of  absorbent  pores  for  the  entrance 
of  liquid,  and  of  several  main  channels  for  its  discharge. 
The  apparatus,  however,  is  not  marked  off  from  the 
general  cavity  of  the  body,  and  digestion  is  not  distinct 
from  circulation.90 

The  sea  urchin  presents  us  with  an  important  advance 
—  one  cavity  with  two  orifices ;  and  the  complicated 
apparatus  of  higher  animals  is  but  the  development  of 
this  type.  This  alimentary  canal  begins  in  a  mouth 
well  provided  with  teeth  and  muscles,  and  extends 
spirally  to  its  outlet,  which  generally  opens  » on  the 
upper,  or  opposite,  surface.  Moreover,  while  in  some 
of  the  worms  the  canal  is  a  simple  tube  running  through 
the  axis  of  the  cylindrical  body  from  oral  orifice  to  anal 
aperture,  the  canal  of  the  sea  urchin  shows  a  distinction 
of  parts,  foreshadowing  the  pharynx,  gullet,  stomach, 
and  intestine.  Both  mouth  and  vent  have  muscles  for 


THE   ALIMENTARY   CANAL 


2/9 


constriction  and  expansion  ;  and,  as  the  vent  is  on  the 

summit  of   the   shell,  and   the  latter   is   covered   with 

spines,     the     ejected 

particles  are  seized  by 

delicate  forks  (pedicel- 

larice\  and  passed  on 

from  one  to  the  other 

down  the  side  of  the 

body,    till     they     are 

dropped   off  into  the 

water.91 

The  worms  present 
us  with  a  great  range 

Of       Structure      in       the 

digestive  tract.     It  is 
sometimes    almost   as 

Simple    aS    that    Of    the 

a a      mere      SaC. 

f^rt-h  wnrm    hv<z   x 

n  nas  a 

tube  running  straight 
through  the  body,  di- 
vided into  pharynx,  esophagus,  crop,  gizzard,  and  saccu- 
lated  intestine  (Fig.  52).  The  leech  has  large  sacs  on 
each  side  of  the  intestine.  The  sea  worms,  like  Nereis, 
have  the  pharynx  armed  with  teeth,  and  some  have 
glandular  coeca  attached  to  the  intestine.  The  plan  is 
that  of  a  straight  tube  extending  from  mouth  to  anus. 
In  myriapods  and  larvae  of  insects,  the  same  general 
plan  is  continued,  the  canal  passing  in  a  straight  line 
from  one  extremity  to  the  other,  but  showing  a  division 
into  gullet,  stomach,  and  intestine.92  Crustacea,  like 
the  lobster,  have  a  short  gullet  leading  to  a  large 
cavity,  situated  in  the  front  of  the  animal,  which  is  a 
gizzard,  rather  than  stomach,  as  it  has  thick  muscular 
walls  armed  with  teeth.  A  well-marked  constriction 


FIG.  237. — Diagrammatic  Section  of  a  Sea  Urchin 
(Echinus):  a,  mouth;  b,  esophagus;  c,  stom- 
ach; d,  intestine;  ft  madreporiform  tubercle; 
g,  stone  canal;  h,  ambulacral  ring;  k,  Polian 
vesicles,  which  are  probably  reservoirs  of  fluid; 
m,  ambulacral  tube;  o,  anus;  p,  ambulacra,  with 
their  contractile  vesicles;  r,  nervous  ring  around 
the  gullet;  s,  two  nervous  trunks,  the  right  termi- 
nating, at  anal  pole,  in  an  eye;  t,  blood-vascular 
rings  connected  by  v,  the  intestinal  blood  vessel; 
iv,  two  arterial  trunks  radiating  from  the  anal 
ring;  x,  an  ovary  opening  at  the  anal  pole  in  a 
genital  plate,  y;  z,  spines,  with  their  tubercles. 


280 


COMPARATIVE   ZOOLOGY 


separates  this  organ  from  the  intestine.     The  "  liver," 
really   a    pancreas,   is    highly   developed  ;    instead    of 

numerous  folli- 
cles, there  is  a 
large  bilaterally 
symmetrical  or- 
gan, divided  into 
three  lobes  on 
each  side,  pour- 
ing its  secretion 
into  the  upper 
part  of  the  intes- 
tine, which  is  the 
true  stomach. 

Among  insects, 
there  is  great 
variation  in  the 
form  and  length 
of  the  canal.  The 
following  parts 
can  generally  be 
distinguished  : 
gullet,  crop,  giz- 
zard, stomach, 
and  large  and 
small  intestines, 
with  many  gland- 
ular appendages. 
The  crop,  gizzard, 

,      iQrfr_      inf^c 
dn          Idrge 

tmg        are        SOITl€- 

timCS          absent, 
.     ,, 

especially  in  the 
carnivorous  species.  In  bees,  the  crop  is  called  the 
"honey-bag.'-'  The  gizzard  is  found  in  insects  having 


FIG.  238.  —  Anatomy  of  a  caterpillar  :  g,  h,  esophagus;  h, 
/.stomach;  k,  hepatic  vessels;  /,  m,  intestine;  q,  r, 
salivary  glands;  /,  salivary  duct;  a,  b,  c,  longitudinal 
tracheal  trunks;  d,  e,  air  tubes  distributed  to  the  vis- 
cera; f,  fat  mass;  v,  x,y,  silk  secretors;  z,  their  excre- 
tory  ducts,  terminating  in  t,  the  spinneret,  or  fusulus. 


THE   ALIMENTARY   CANAL 


28l 


mandibles,  and  is  frequently  lined  with  rows  of  horny 
teeth,  which  are  specially  developed  in  grasshoppers, 
crickets,  and  locusts.  The  intestines  are  remarkable 


:       a\ 


FIG.  239.  —  Alimentary  canal  of  a  Beetle:  FIG.  240.  —  Alimentary    Canal    of    the    Bee 

a,  pharynx;  b,  gullet,  leading  to  crop,  (Apis  mellifica}\  a,  gullet;  b,  crop;  c,  d, 

c,  gizzard,  d,  and  stomach,  e;  f,  deli-  stomach;    e,  small   intestine;  f,  large  in- 

cate    urinary    tubes;    gs    intestine;    h,  testine;  g,  anal  orifice;  /*,  urinary  vessels; 

other  secreting  organs.  /,  auxiliary  glands. 

for  their  convolutions.  Insects  have  no  true  liver ;  but 
its  functions  are  performed  by  little  cell  masses  on  the 
inside  of  the  stomach.93 


FIG.  241.  —  Anatomy  of  a  Sphinx  Moth;  «,  nervous  cord;  «',  brain  sending  ofT  nerves 
to  the  legs,  /',  I" ,  /'",  and  for  the  wings  at  «";  h,  dorsal  vessel,  or  heart;  c,  crop; 
s,  stomach;  it  intestines;  o,  reproductive  organs;  o' ,  oviduct;  8-20,  segments. 

The  alimentary  canal  of  spiders  is  short  and  straight, 
the  pharynx  and  gullet  being  very  minute.     The  stom- 


282 


COMPARATIVE   ZOOLOGY 


ach  is  characterized  by  sending  out  tubular  prolonga- 
tions, and  the  intestine  ends  in  a  large  bladderlike 
expansion.  Scorpions  have  no  stomachal  cavity — a 
straight  intestine  passes  directly  through  the  body. 

In  bivalve  mollusks,  like  the  clam,  the  mouth  opens 
into  a  short  esophagus  which  leads  into  the  stomach, 

which  lies  em- 
bedded in  a  large 
liver,  and  the  in- 
testine, describ- 
ing a  few  turns, 
passes  directly 
through  the 
heart.94  In  the 
univalve  mol- 
lusks, like  the 
snail,  the  gullet 
is  long  and  fre- 
quently expands 
into  a  crop ;  the 
stomach  is  often 
double,  the  ante- 
rior being  a  giz- 
zard provided 
with  teeth  for 
mastication ;  the  intestine  passes  through  the  liver,  and 
ends  in  the  fore  part  of  the  body,  usually  on  the  right 
side. 

The  highest  mollusks,  as  the  cuttlefish  and  nautilus, 
exhibit  a  marked  advance.  A  mouth  with  powerful 
mandibles  leads  to  a  long  gullet,  which  ends  in  a  strong 
muscular  gizzard  resembling  that  of  a  fowl.95  Below 
this  is  a  cavity,  which  is  either  a  stomach  or  duodenum ; 
it  receives  the  secretion  from  a  large  digestive  gland  or 
pancreas.  The  intestine  is  a  tube  of  uniform  size,  which, 


FIG.  242.  —  Alimentary  Canal  of  the  Oyster:  a,  stomach 
laid  open;  d,  liver;  b,  c,  d,f,  convolutions  of  the  intes- 
tine; g,  anal  aperture;  n,  o,  auricle  and  ventricle;  /,  m, 
adductor  muscle;  h,  k,  lobes  of  mouth  divided  to  show 
the  venous  canals  at  the  base  of  the  gills. 


THE    ALIMENTARY   CANAL 


283 


FIG.  243. — Anatomy  of  a  Gastropod  (Snail):  a,  mouth;  b,  foot;  c,  anus;  d,  lung;  e, 
stomach,  covered  above  by  the  salivary  glands;  f,  intestine;  g,  "liver";  h,  heart; 
f,  aorta;  j,  gastric  artery;  k,  artery  of  the  foot;  /,  hepatic  artery:  mt  abdominal 
cavity,  supplying  the  place  of  a  venous  sinus;  «,  irregular  canal  communicating 
with  the  abdominal  cavity,  and  carrying  the  blood  to  the  lung;  o,  Vessel  carrying 
blood  from  the  lung  to  the  heart. 


q 


FIG.  244.  —  Anatomy  of  a  Lamellibranch  (Mactra):  a,  shell;  b,  mantle;  c,  tentacles,  or 
lips;  d,  mouth;  e,  nerves;  f,  muscles;  g,  anterior,  and  n,  posterior  ganglion;  k, 
"liver";  z',  heart;  k,  stomach;  /,  intestine  passing  through  the  heart;  m,  kidney; 
0,  anal  end  of  the  intestine;  /,  exhalent,  and  q,  inhalent  respiratory  tubes,  or 
siphons;  r,  gills;  s,  foot. 


284 


COMPARATIVE   ZOOLOGY 


after  one  or  two  slight  curves,  bends  up,  and  opens  into 
the  "  funnel "  near  the  mouth. 

Fishes  have  a  simple,  short,  and  wide  alimentary 
canal.  The  stomach  is  separated  from  the  intestine  by 
a  narrow  "  pyloric  "  orifice,  or  valve,  but  is  not  so  clearly 
distinguished  from  the  gullet,  so  that  regurgitation  is 

easy.96  Indeed,  it  is  common 
for  fishes,  to  disgorge  the  in- 
digestible parts  of  their  food, 
and  some,  as  the  carp,  send 
the  food  back  to  the  pharynx 
to  be  masticated.  The  stom- 
ach  is  usually  bent,  like  a 
siphon ;  but  the  intestine  is 
nearly  straight,  and  without 
any  marked  distinction  into 
small  and  large.  Its  appen- 
dages are  a  large  liver  and  a 
rudimentary  pancreas. 

In  the  amphibians,  as  the 

FIG.  245. —  Anatomy  of  a  Cephalopod  IT. 

(diagram):    a,   tentacles;    6,   masti-     frOgS,  the    dlgCStlVC   apparatus 

Z?jSS*?~S?.  ^i  is  vefy sirailar  to  that  of  fishes ; 

/,  esophagus;    g,  internal  shell,  or     ^ut    the     tWO     portions     of     the 
cuttle-bone     ;    h,  stomach;    z,  in-  x 

testine;  k,  anus;  /,  funnel;   m,   intestine  can  be  more  readily 

ink  bag;    n,  ovary;    o,  oviduct;  /,       T    .•  •    -,       •>  ^r-i  ^-i 

"liver";    r,  gill   contained   in   the     distinguished.          The      reptiles 

™s;£n«  "!   generally  have   a   long,  wide 

gullet,  which  passes  insensibly 

into  the  stomach,  and  a  short  intestine  (about  twice  the 
length  of  the  body)  very  distinctly  divided  into  small 
and  large  by  a  constriction.97  The  vegetable-feeding 
tortoises  have  a  comparatively  long  intestinal  tube ;  and 
the  serpents  have  a  slender  stomach,  but  little  wider 
than  the  rest  of  the  alimentary  canal. 

The  stomach  of  the  crocodile  (Fig.  247)  is  more  com- 
plex than  any  hitherto  mentioned.  "  It  resembles  that  of 


THE   ALIMENTARY   CANAL 


285 


the  cuttlefish,  but  offers  a  still  more  striking  analogy  to 
the  gizzard  of  a  bird,  having  very  thick  walls,  and  the 
muscular  fibers  radiating  precisely  in  the  same  manner, 


FIG.  246.  —  Anatomy  of  the  Carp:  br,  branchiae,  or  gills;  c,  heart;  ft  liver;  v.n,  v.n'. 
swimming  bladder;  c.i,  intestinal  canal;  0,  ovarium;  u,  ureter;  a,  anus;  o' ',  gen- 
ital opening;  «',  opening  of  ureter.  The  side  view  shows  the  disposition  of  the 
muscles  in  vertical  flakes. 


286 


COMPARATIVE   ZOOLOGY 


so  that,  in  this  respect,  the  crocodile  may  be  considered 
to   be   intermediate   between   reptiles  and   birds.98     In 

crocodiles  also 
the  duodenum, 
with  which  the 
intestine  begins, 
is  first  distinctly 
defined.  Into 
this  part  of  the 
intestine  the 
liver  and  pan- 
creas, or  sweet- 
bread, pour 
their  secretions. 
Furthermore,  in 
the  lower  ani- 
mals, the  intes- 
tines lie  more 
or  less  loose  in 

the  abdomen  ; 
-i  .  •  f-L.  „  r^-r^r* 

odile,   and   like- 

wise in  birds  and  mammals,  they  are  supported  by  a 
membrane  called  mesentery. 

In  birds,  the  length  of  the  alimentary  canal  varies 
with  their  diet,  being  greatest  in  those  living  on  grain 
and  fruit.  The  gullet  corresponds  in  length  with  the 
neck,  which  is  longest  in  the  long-legged  tribes,  and  in 
width  with  the  food.  In  those  that  swallow  large  fish 
entire,  the  gullet  is  dilatable,  as  in  snakes.  In  nearly 
all  birds,  the  food  is  delayed  in  some  cavity  before 
digestion  :  thus,  the  pelican  has  a  bag  under  the  lower 
jaw,  and  the  cormorant  has  a  capacious  gullet,  where  it 
stores  up  fishes  ;  while  those  that  gorge  themselves  at 
intervals,  as  the  vulture,  or  feed  on  seeds  and  grains, 


FIG.  247.  —  Stomach  of  the  Crocodile:  a,  muscular  fibers  ra- 
dialing  from  a  central  tendon,  b;  d,  commencement  of 
duodenum;  c,  esophagus;  /,  intestine. 


THE   ALIMENTARY   CANAL 


287 


as  the  turkey,  have 
near  the  lower  end 
of  the  gullet"  The 
ostrich,  goose,  swan, 
most  of  the  waders, 
and  the  fruit  or 
insect-eating  birds, 
which  find  their 
food  in  tolerable 
abundance,  and  take 
it  in  small  quanti- 
ties, have  no  such 
reservoir.  Pigeons 
have  a  double  crop. 
In  all  birds,  the 
food  passes  from 
the  gullet  into  the 
proventriculus,  or 
stomach  proper, 
where  it  is  mixed 
with  a  "  gastric 
juice"  secreted 
from  glands  on  the 
surface.  Thence  it 
goes  into  the  giz- 
zard, an  oval  sac 
of  highly  muscular 
texture,  and  lined 
with  a  tough,  horny 
skin.100  The  giz- 
zard is  most  highly 
developed,  and  of  a 
deep-red  color,,  in 
the  scratchers  and 
flat-billed  swimmers 


a  pouch,  called  the  crop,  developed 


FIG.  248.  —  Digestive  Apparatus  of  the  Fowl:  i, 
tongue;  2,  pharynx;  3,  5,  esophagus;  4,  crop; 
6,  proventriculus;  7,  gizzard;  8,  9,  10,  duodenum; 
n,  12,  small  intestine;  13,  two  caeca  (analogue  of 
the  colon  of  mammals') ;  14,  their  insertion  into  the 
intestinal  tube;  15,  rectum;  16,  cloaca;  17,  anus; 
18,  mesentery;  19,  20,  left  and  right  lobes  of  liver; 
21,  gall  bladder;  22,  insertion  of  pancreatic  and 
biliary  ducts;  23,  pancreas;  24,  lung;  25,  ovary; 
26,  oviduct. 

(as  fowls  and  swans) ;  but  compara- 


288 


COMPARATIVE   ZOOLOGY 


tively  thin  and  feeble 
in  birds  of  prey  (as  the 
eagle).  The  gizzard  is 
followed  by  the  intes- 
tines, which  are  longer 
than  those  of  reptiles : 
the  small  intestine  be- 
gins with  a  loop  (the 
duodenum),  and  is 
folded  several  times 
upon  itself ;  the  large 
intestine  is  short  and 
straight,  terminating  in 
the  sole  outlet  of  the 
body,  the  cloaca.  A 
liver  and  pancreas  are 
always  attached  to  the 
upper  part  of  the  small 
intestine. 

The  alimentary  canal 
in  mammals  is  clearly 
separated  into  four  dis- 
tinct cavities :  the 
pharynx,  or  throat;  the 
esophagus,  or  gullet ; 
the  stomach  ;  and  the 
intestines. 

The  pharynx  is  more 
complicated  than  in 
birds.  It  is  a  funnel- 


FlG.  249.  —  Digestive  Apparatus  of 
Man  (diagram):  i,  tongue;  2,  phar- 
ynx; 3,  esophagus;  4,  soft  palate;  5,  larynx;  6,  palate;  7,  epiglottis;  8,  thyroid  cartilage; 
9,  beginning  of  spinal  marrow;  10,  n,  12,  vertebrae,  with  spinous  processes;  13,  cardiac 
orifice  of  stomach;  14,  left  end  of  stomach;  18,  pyloric  valve;  19,  20,  21,  duodenum; 
22,  gall  bladder;  27,  duct  from  pancreas;  28,  29,  jejunum  of  intestine;  30,  ileum;  34,  cce- 
cum;  36,  37,  38,  colon,  or  large  intestine;  40,  rectum. 


THE   ALIMENTARY   CANAL  289 

shaped  bag,  having  seven  openings  leading  into  it :  two 
from  the  nostrils,  and  two  from  the  ears ;  one  from  the 
windpipe,  guarded  by  the  epiglottis ;  one  from  the  mouth, 
with  a  fleshy  curtain  called  the  soft  palate  ;  and  one  from 
the  esophagus.  It  is  the  natural  passage  for  food  be- 
tween the  mouth  and  the  esophagus,  and  of  air  between 
the  nostrils  and  windpipe.  Like  the  mouth,  it  is  lined 
with  a  soft  mucous  membrane. 

The  esophagus  is  a  long  and  narrow  tube,  formed  of 
two  muscular  layers :  in  the  outer  layer,  the  fibers  run 
lengthwise  ;  in  the  other,  they  are  circular.  It  is  lined 


FIG.  250.  —  Ideal  Section  of  a  Mammalian  Vertebrate:  A,  pectoral,  or  fore  limb;  B, 
pelvic,  or  hind  limb;  a,  mouth;  b,  cerebrum;  c,  cerebellum;  d,  nose;  e,  eye;  ft 
ear;  g,  esophagus;  h,  stomach;  i,  intestine;  /,  diaphragm,  or  midriff;  k,  rectum, 
or  termination  of  intestine;  /,  anus;  m,  liver;  «,  spleen;  o,  kidney;  /,  sympathetic 
system  of  nerves;  q,  pancreas;  r,  urinary  bladder;  s,  spinal  cord;  u,  ureter;  v, 
vertebral  column;  -w,  heart;  x,  lung;  y,  trachea,  or  windpipe;  z,  epiglottis. 

with  mucous  membrane.  While  in  all  fishes,  reptiles, 
and  birds  the  body  cavity  is  one,  in  mammals  it  is 
divided,  by  a  partition  called  the  diaphragm,  into  two 
cavities,  —  the  thorax,  containing  the  heart,  lungs,  etc. ; 
and  the  abdomen,  containing  the  stomach,  intestines,  etc. 
The  esophagus  passes  through  a  slit  in  the  diaphragm, 
and  almost  immediately  expands  into  the  stomach. 

In  the  majority  of  mammals,  the  stomach  is  a  muscu- 
lar bag  of  an  irregular  oval  shape,  lying  obliquely  across 
the  abdomen.     In  the  flesh  eaters,  whose  food  is  easy 
DODGE'S  GEN.  ZOOL. —  19 


2QO 


COMPARATIVE   ZOOLOGY 


of  solution,  the  stomach  is  usually  simple,  and  lies  nearly 
in  the  course  of  the  alimentary  canal ;  but  in  proportion 
as  the  food  departs  more  widely  in  its  composition  from 
the  body  itself,  and  is  therefore  more  difficult  to  digest, 
we  find  the  stomach  increasing  in  size  and  complexity, 


FIG.  251.  — Section  of  Horse's  Stomach;  A,  FlG.  252.  — Stomach     of     the 

left  sac;  B,  right  sac;   C,  duodenum.  Porpoise:    c,  cardiac  open- 

ing; /,  pyloric  opening. 

and  turned  aside  from  the  general  course  of  the  canal, 
so  as  to  retain  the  food  a  longer  time.  The  inlet  from 
the  esophagus  is  called  cardiac  opening;  the  outlet 
leading  into  the  intestines  is  called  pyloric  opening. 

In  the  carnivores, 
apes,  and  most  odd- 
toed  quadrupeds,  the 
stomach  resembles 
that  of  man.  That 
of  the  toothless  ant- 
eater  has  the  lower 
part  turned  into  a 

FIG.  253.—  Stomach  of  the  Lion:  c,  cardiac  orifice,  or      kind     of    gizzard    for 
f  esophagus;  A  pyloric  orifice.  crushing        its       fo^ 

The  elephant's  is  subdivided  by  numerous  folds.  In  the 
horse,  it  is  constricted  in  the  middle ;  and  in  the  rodents, 
porpoises,  and  kangaroos,  the  constriction  is  carried  so 


THE   ALIMENTARY   CANAL  291 

far  as  to  make  two  or  three  sections.  But  animals 
that  chew  the  cud  (ruminants)  have  the  most  complex 
stomach.  It  is  divided  into  four  peculiar  chambers : 
First,  the  paunch  (rumen),  the  largest  of  all,  receives 
the  half-masticated  food  when  it  is  first  swallowed. 
The  inner  surface  is  covered  with  papillae,  except  in 
the  camel,  which  has  large  cells  for  storing  up  water. 
From  this,  the  food  passes  into  the  honeycomb  stomach 
(reticulum),  so  named  from  its  structure.  Liquids  swal- 
lowed usually  go  directly  to  this  cavity,  without  passing 
through  the  paunch,  and  hence  it  is  sometimes  called 
the  water  bag.  Here  the  food  is  made  into  little  balls, 


FIG.  254.  —  Complex  Stomach  of  a  Ruminant:  a,  gullet;  b,  rumen,  or  paunch;  c,  reticu- 
lum ;  d,  psalterium,  or  manyplies ;  e,  abomasus ;  _/",  pylorus  leading  to  duodenum. 

and  returned  to  the  mouth  to  undergo  a  thorough  mas- 
tication. When  finally  swallowed,  it  is  directed,  by  a 
groove  from  the  esophagus,  to  the  third,  and  smallest, 
cavity,  the  manyplies  (psalterium),  named  from  its 
numerous  folds,  which  form  a  strainer  to  keep  back 
any  undivided  food ;  and  thence  it  passes  into  the  true 
stomach  (abomasus),  from  which,  in  the  calf,  the  rennet  is 
procured  for  curdling  milk  in  the  manufacture  of  cheese. 
This  fourth  cavity  is  like  the  human  stomach  in  form 
and  function,  and  is  the  only  part  which  secretes  gastric 
juice.  The  rumen  and  reticulum  are  rather  dilatations 
of  the  esophagus  than  parts  of  the  stomach  itself  ;  while 
the  latter  is  divided  by  constriction  into  two  chambers, 
the  psalterium  and  abomasus,  as  in  many  other  animals. 


292 


COMPARATIVE   ZOOLOGY 


nun. 


In  structure  the  stomach  resembles  the  esophagus. 
The  smooth  outside  coat  {peritoneum)  is  a  reflection  of 
the  membrane  which  lines  the  whole  abdomen.  The 
middle,  or  muscular,  coat  consists  of  three  layers  of 
fibers,  running  lengthwise,  around  and  obliquely.  The 
successive  contraction  and  relaxing 
of  these  fibers  produce  the  worm- 
like  motion  of-  the  stomach,  called 
peristaltic.  The  innermost,  or  mu- 
cous, membrane,  is  soft,  velvety,  of 
a  reddish  gray  color  in  man,  and 
filled  with  multitudes  of  glands, 
which  secrete  the  gastric  juice. 
The  human  stomach,  when  dis- 
tended, will  hold  about  five  pints ; 
that  of  the  kangaroo  is  as  long  as 
its  body. 

The  intestinal  canal  in  mammals 
begins  at  the  pyloric  end  of  the  stom- 
ach, where  there  is  a  kind  of  valve  or 
circular  muscle.     Like  the  stomach, 
FIG  255 -vertical   section  ft  varies  greatly,  according  to  the 

of  the  Coats  of  the  Stomach :  &  J  ' 

d,  surface  of  mucous  mem-  nature  of  the  food.     It  is  generally 

brane,  and  mouths  of  gastric     ,  ,    .       ,,  -     it      r        i  i 

follicles;  m,  gastric  tubuii,   longest  m  the  vegetable  feeders,  and 

or  follicles;  mm,  dense  con-     shortest    in    the    flesh    feeders.        The 
nective  tissue;  sm,  sub-mu- 
cous tissue;  cm, transverse   greater  length  in  the  former  is  due 

muscular  fiber;   lmt  longi- 
tudinal   muscular   fibers;    to  the  fact  that  vegetable  food  re- 

s,  fibrous,  or  serous,  coat.        ^.^    ^    ^^    ^    f()r    digestion> 

and  that  a  greater  bulk  of  such  food  is  required  to 
obtain  a  given  quantity  of  nutriment.  The  intestines 
measure  150  feet  in  a  full-grown  ox,  while  they  are 
but  three  times  the  length  of  the  body  in  the  lion, 
and  six  times  in  man.  Save'  in  some  lower  forms,  as 
the  whales,  there  are  two  main  divisions,  the  "small" 
and  "large"  intestines,  at  the  junction  of  which  is  a 


THE   ALIMENTARY   CANAL  293 

valve.  The  former  is  the  longer  of  the  two,  and  in  it 
digestion  is  completed,  and  from  it  the  most  of  absorp- 
tion takes  place.  The  large  intestine  is  mainly  a  tem- 
porary lodging  place  for  the  useless  part  of  the  food, 
until  it  is  expelled  from  the  body.  The  beginning  of 
the  small  intestine  is  called  the  duodenum,  into  which 
the  ducts  from  the  liver  and  pancreas  open.  The  in- 
testinal canal  has  the  same  structure  as  the  stomach, 
and  by  a  peristaltic  motion  its  contents  are  propelled 
downward.  The  inside  of  the  small  intestine  is  covered 
with  a  host  of  threadlike  processes  (villi\  resembling 
the  pile  of  velvet. 

In  taking  this  general  survey  of  the  succession  of 
forms  which  the  digestive  apparatus  presents  among  the 
principal  groups  of  animals,  we  cannot  fail  to  trace  a 
gradual  specialization.  First,  a  simple  sac,  one  orifice 
serving  as  inlet  for  food  and  outlet  for  indigestible 
matter;  next,  a  short  tube,  with  walls  of  its  own  sus- 
pended in  the  body  cavity ;  then  a  canal  passing  through 
the  body,  and,  therefore,  having  both  mouth  and  vent ; 
next,  an  apparatus  for  mastication,  and  a  swelling  of 
the  central  part  of  the  canal  irito  a  stomach,  having  the 
special  endowment  of  secreting  gastric  juice ;  then  a 
distinction  between  the  small  and  large  intestine,  the 
former  thickly  set  with  villi,  and  receiving  the  secretions 
of  large  glands.  We  also  notice  that  food,  the  means 
of  obtaining  it,  the  instruments  for  mastication,  and 
the  size  and  complexity  of  the  alimentary  canal,  are 
closely  related. 


CHAPTER    XIII* 
HOW  ANIMALS   DIGEST 

The  Object  of  the  Digestive  Process  is  the  reduction 
of  food  into  such  a  state  that  it  can  be  absorbed  into  the 
system.  For  this  purpose,  if  solid,  it  is  dissolved ;  for 
fluidity  is  a  primary  condition,  but  not  the  only  one. 
Many  soluble  substances  have  to  undergo  a  chemical 
change  before  they  can  form  parts  of  the  living  body. 
If  albumen  or  sugar  be  injected  into  the  veins,  it  will 
not  be  assimilated,  but  be  cast  out  unaltered. 

To  produce  these  two  essential  changes,  solution  and 
transmutation,  two  agencies  are  used  —  one  mechanical, 
the  other  chemical.  The  former  is  not  always  needed, 
for  many  animals  find  their  food  already  dissolved,  as 
the  butterfly ;  but  solid  substances,  to  facilitate  their 
solution,  are  ground  or  torn  into  pieces  by  teeth,  as  in 
man ;  by  jaws,  as  in  the  lobster;  or  by  a  gizzard,  as  in 
the  turkey. 

The  chemical  preparation  of  food  is  indispensable.101 
It  is  accomplished  by  one  or  more  solvent  fluids  secreted 
in  the  alimentary  canal.  The  most  important,  and  one 
always  present,  is  the  gastric  juice,  the  secretion  of 
which  is  restricted  to  the  stomach,  when  that  cavity 
exists.  In  the  higher  animals,  numerous  glands  pour 
additional  fluids  into  the  digestive  tube,  as  saliva  into 
the  upper  part  or  mouth,  and  bile  and  pancreatic  juice 
into  the  upper  part  of  the  intestine.  In  fact,  the  mucous 

*  See  Appendix. 
294 


HOW  ANIMALS    DIGEST  295 

membrane,  which  lines  the  alimentary  canal  throughout, 
abounds  with  secreting  glands  or  cells. 

The  Digestive  Process  is  substantially  the  same  in-  all 
animals,  but  it  is  carried  farther  in  the  more  highly  de- 
veloped forms.  In  the  Infusoria,  the  food  is  acted  upon 
by  some  secretion  from  the  protoplasm  of  the  body,  the 
exact  nature  of  which  is  unknown.  In  the  starfish  and 
sea  urchin,  we  find  two  solvents  —  a  gastric  juice,  and  an- 
other resembling  pancreatic  juice;  but  the  two  appear 
to  mingle  in  the  stomach.  Mollusks  and  arthropods 
show  a  clear  distinction  between  the  stomach  and  intes- 
tine, and  the  contents  of  the  pancreas  are  poured  into 
the  latter.  There  are,  therefore,  two  stages  in  the  digest- 
ive act :  first,  the  food  is  dissolved  by  the  gastric  juice  in 
the  stomach,  forming  chyme  ;  secondly,  the  chyme,  upon 
entering  the  intestine,  is  changed  into  chyle  by  the  action 
of  the  pancreatic  secretion,  and  is  then  ready  to  be 
absorbed  into  the  system. 

In  vertebrates,  a  third  solvent  is  added,  the  bile,  which 
aids  the  pancreatic  juice  in  completing  digestion.  But 
mammals  and  insects  have  a  still  more  perfect  and  elab- 
orate process ;  for  in  them  the  saliva  of  the  mouth  acts 
chemically  upon  the  food  ;  while  the  saliva  in  many  other 
animals  has  no  other  office,  so  far  as  we  know,  than  to 
moisten  the  food  for  swallowing. 

Taking  man  as  an  example,  let  us  note  the  main  facts 
in  the  process.  During  mastication,  by  which  the  rela- 
tive surface  is  increased,  the  food  is  mixed  with  saliva, 
which  moistens  it,102  and  turns  a  small  part  of  the 
s(tarch  into  grape  sugar.  Passed  into  the  stomach,  the 
food  meets  the  gastric  juice.  This  is  acid,  and,  first, 
stops  the  action  of  the  saliva  ;  secondly,  by  means  of 
the  pepsin  which  it  contains,  and  the  acid,  it  dissolves 
the  albumen,  fibrin,  and  other  such  constituents  of  the 
food.  This  solution  of  albuminoids  is  called -a  peptone, 


296  COMPARATIVE   ZOOLOGY 

and  is  especially  distinguished  from  other  such  solutions 
by  its  diff usibility  —  i.e.,  the  ease  with  which  it  passes 
through  a  membrane.  Some  of  these  peptones,  with 
the  sugars  of  the  food,  whether  original  or  the  product 
of  the  action  of  the  saliva,  are  absorbed  from  the  stomach. 
The  food,  while  in  the  stomach,  is  kept  in  continual 
motion,  and,  after  a  time,  is  discharged  in  gushes  into 
the  intestine.  The  name  chyme  is  given  to  the  pulpy 
mass  of  food  in  the  stomach.103  In  the  intestine  the 
chyme  meets  three  fluids  —  bile,  pancreatic  juice,  and 
intestinal  juice.  All  of  these  are  alkaline,  and  at  once 
give  the  acid  chyme  an  alkaline  reaction.  This  change 
permits  the  action  of  the  saliva  to  recommence,  which 
is  aided  by  the  pancreatic  and  intestinal  juices.  The 
pancreatic  juice  has  much  more  important  functions. 
It  changes  albuminoid  food  into  peptones,  and  probably 
breaks  up  the  fats  into  very  small  particles,  which  are 
suspended  in  the  fluid  chyle.  This  forms  an  emulsion, 
like  milk,  and  causes  the  chyle  to  appear  whitish.  The 
bile  has  important  functions,  but  little  understood.  It 
emulsifies  and  saponifies  part  of  the  fats,  so  that  they 
are  dissolved,  and  perhaps  aids  in  preventing  the  food 
from  decomposing  during  the  process  of  digestion  and 
absorption.  The  chyle  is  slowly  driven  through  the 
small  intestine  by  the  creeping,  peristaltic  motion  of  its 
walls,104  the  nutritious  portion  being  taken  up  by  the 
absorbents,  as  described  in  the  next  chapter,  while  the 
undigested  part  remaining  is  discharged  from  the  large 
intestine.105  * 


CHAPTER   XIV 


THE  ABSORBENT   SYSTEM 

THE  nutritive  matter  (chyle),  prepared  by  the  digestive 
process,  is  still  outside  of  the  organism.  How  shall  it 
enter  the  living  tissue  ? 

In  animals,  like  the 
Infusoria  and  polyps, 
whose  digestive  de- 
partment is  not  sepa- 
rated from  the  body 
cavity,  the  food,  as 
soon  as  dissolved,  min- 
gles freely  with  the 
parts  it  has  to  nourish. 
In  the  higher  inverte- 
brates having  an  ali- 
mentary canal,  the 
chyle  passes,by  simple 
transudation,  through 
the  walls  of  the  canal 
directly  into  the  soft 
tissues,  as  in  insects,  or 
is  absorbed  from  the 
canal  by  veins  in  con- 
tact with  it,  as  in  sea 
urchins,  mollusks. 
worms,  and  crusta- 
ceans, and  then  dis- 
tributed through  the 
body. 


FIG.  256.  —  Section  of  Injected  Small  Intestine  of  Cat: 
a,  b,  mucosa;  g,  villi;  i,  their  absorbent  vessels; 
h,  simple  follicles;  c,  muscularis  mucosae;  d,  sub- 
mucosa;  e,  e' ,  circular  and  longitudinal  layers  of 
muscle;  f,  fibrous  coat.  All  the  dark  lines  represent 
blood  vessels  filled  with  an  injection  mass. 


297 


298 


COMPARATIVE   ZOOLOGY 


In  vertebrates  only  do  we  find  a  special  absorbent  sys- 
tem. Three  sets  of  vessels  are  concerned  in  the  general 
process  by  which  fresh  material  is  taken  up  and  added 
to  the  blood :  Capillaries,  Lacteals,  and  Lymphatics. 
Only  the  two  former  draw  material  from  the  alimentary 
canal. 

The  food  probably  is  absorbed  almost  as  fast  as 
it  is  dissolved,  and,  therefore,  there  is  a  constant  loss  in 
the  passage  down  the  canal.  In  the  mouth  and  esoph- 
agus, the  absorption  is  slight ;  but  much  of  that  which 
has  yielded  to  the  gastric  juice,  with  most  of  the  water, 
is  greedily  absorbed  by  the  capillaries  of  the  stomach, 
and  made  to  join  the  current  of  blood  which  is  rushing 
to  the  liver.  Absorption  by  the  capillaries  also  takes 
place  from  the  skin  and  lungs.  Medicinal  or  poisonous 
gases  and  liquids  are  readily  introduced  into  the  system 
by  these  channels. 

We  have  seen  that  the  oily  part  of  the  food  passes 
unchanged  from  the  stomach  into  the  small  intestine, 

where,  acted  upon  by  the 
pancreatic  juice,  it  is  cut 
up  into  extremely  minute 
particles,  and  that  the  un- 
digested albuminoids  and 
starches  are  digested  in 
the  intestine.  Two  kinds 
of  absorbents  are  present 
in  the  intestine,  lacteals 
and  blood  capillaries.  Both 
the  lymphatic  and  blood 
systems  send  vessels  into 
the  velvety  villim  with 
which  the  -  intestine  is 
lined.  The  blood  capillaries  lie  toward  the  outside  of 
the  villus  and  the  lacteal  in  the  center.  The  albumi- 


FIG.  257.  —  Lacteal  System  of  Mammal:  a, 
descending  aorta,  or  principal  artery;  b, 
thoracic  duct;  c,  origin  of  lacteal  vessels, 
g,  in  the  walls  of  the  intestine,  d;  e, 
mesentery,  or  membrane  attaching  the 
intestine  to  walls  of  the  body;  /,  lacteal, 
or  mesenteric,  glands. 


THE  ABSORBENT   SYSTEM 


299 


noids  and  sugars  are  chiefly  absorbed  by  the  blood  vessels 
and  go  to  the  liver.  The  fats  pass  on  into  the  lacteals, 
which  receive  their  name  from  the  milky  appearance 
of  the  chyle.  These  lacteals  unite  into  larger  trunks, 
which  lie  in  the  mesentery 
(or  membrane  which  sus- 
pends the  intestine  from 
the  back  wall  of  the  ab- 
domen), and  these  pour 
their  contents  into  one 
large  vessel,  the  thoracic 
duct,  lying  along  the 
backbone,  and  joining  the 
jugular  vein  in  the  neck. 

The  lacteals  are  only  a 
special  part  of  the  great 
lymphatic  system,  which 
absorbs  and  carries  to  the 
thoracic  duct  matter  from 
all  parts  of  the  body.107 
The  lymph  is  a  transpar- 
ent fluid  having  many 
white  blood  corpuscles. 
It  is,  in  fact,  blood,  minus 
the  red  corpuscles,  while 
chyle  is  the  same  fluid  ren- 
dered milky  by  numerous 

fat  globules.        During  the     FlG.  258._Prindpal  Lymphatics  of  the  Hu- 

intervals  of  digestion,  the     man  Body:  *»  union  of  left  Jusular  and 

subclavian  veins;    b,  thoracic  duct;    c,  re- 
laCteals       Carry       Ordinary        ceptaculum   chyli.      The  oval    bodies  are 

lymph.     This  fluid  is  the 

overflow  of  the  blood  —  the  plasma  and  white  corpuscles 
which  escape  from  the  blood  capillaries,  and  carry  nutri- 
ment to,  and  waste  from,  those  parts  of  the  various  tis- 
sues which  are  not  in  contact  with  the  blood  capillaries. 


300  COMPARATIVE   ZOOLOGY 

This  surplus  overflow  is  returned  to  the  blood  by  the 
lymphatics.  The  current  is  kept  up  by  the  movements 
of  the  body,  and  in  many  vertebrates,  as  frogs  and 
fishes,  by  lymph  hearts. 

Like  the  roots  of  plants,  the  absorbent  vessels  do  not 
commence  with  open  mouths  ;  but  the  fluid  which  enters 
them  must  traverse  the  membrane  which  covers  their 
minute  extremities.  This  membrane  is,  however,  porous, 
and  the  fluids  pass  through  it  by  the  processes  of  filtra- 
tion and  diffusion,  or  dialysis.  How  the  fat  gets  into 
the  lacteals  is  not  yet  well  understood,  but  the  lacteals 
are  themselves  rhythmically  contractile,  and  force  the 
absorbed  chyle  toward  the  heart.  The  valves  of  the 
lymphatics  prevent  its  return. 


CHAPTER   XV* 

THE   BLOOD   OF   ANIMALS 

The  Blood  is  that  fluid  which  carries  to  the  living 
tissues  the  materials  necessary  to  their  growth  and 
repair,  and  removes  their  waste  and  worn-out  material. 
The  great  bulk  of  the  body  is  occupied  with  apparatus 
for  the  preparation  and  circulation  of  this  vital  fluid. 

The  blood  of  the  lower  animals  (invertebrates)  differs 
so  widely  from  that  of  man  and  other  vertebrates,  that 
the  former  were  long  supposed  to  be  without  blood.  In 
them  the  blood  is  commonly  colorless  ;  but  it  has  a  bluish 
cast  in  crustaceans ;  reddish,  yellowish,  or  greenish,  in 
worms ;  and  reddish,  greenish,  or  brownish,  in  jelly- 
fishes.  The  red  liquid  which  appears  when  the  head  of 
a  fly  is  crushed  is  not  blood,  but  comes  from  the  eyes. 
In  vertebrates,  the  blood  is  red,  excepting  the  white- 
blooded,  fishlike  lancelet  Amphioxus.m 

As  a  rule,  the  more  simple  the  fabric  of  the  body,  the 
more  simple  the  nutritive  fluid.  In  unicellular  animals 
(as  Protozoa),  in  those  whose  cells  are  comparatively 
independent  (as  sponges),  and  in  small  and  lowly  organ- 
ized animals  (like  hydra),  there  is  no  special  circulating 
fluid.  Each  cell  feeds  itself  either  directly  from  parti- 
cles of  food,  or  from  the  products  of  digestion.  In 
polyps  and  jellyfishes,  the  blood  is  scarcely  different 
from  the  products  of  digestion,  although  a  few  blood 
corpuscles  are  present.  But  in  the  more  highly  organ- 
ized invertebrates  the  blood  is  a  distinct  tissue,  coagu- 

*  See  Appendix. 
301 


302 


COMPARATIVE   ZOOLOGY 


lating,  and  containing  white  corpuscles.  The  blood  of 
the  vertebrates,  apparently  a  clear,  homogeneous  liquid, 
really  consists  of  minute  grains,  or  globules,  of  organic 
mattep  floating  in  a  fluid.  If  the  blood  of  a  frog  be 
poured  on  a  filter  of  blotting  paper,  a  transparent  fluid 
(called  plasma}  will  pass  through,  leaving  red  particles, 
resembling  sand,  on  the  upper  surface.  Under  the  mi- 
croscope, these  particles  prove  to  be  cells,  or  flattened 
disks  (called  corpuscles^  containing  a  nucleus ;  some  are 
colorless,  and  others  red.  In  mammals  the  red  disks 


E 


FIG.  259.  —  Blood  Corpuscles:  A,  red  corpuscles  in  rouleaux,  a,  a,  colorless  corpuscles, 
magnified  about  400  times;  B,  red  corpuscles  in  focus;  C,  view  of  edge;  D,  three- 
quarters  view;  E,  red  corpuscle  swollen  with  water;  F,  G,  H,  distorted  red  corpuscles. 

have  a  tendency  to  collect  together  into  piles  ;  the  color- 
less ones  remain  single.  Meanwhile,  the  plasma  sepa- 
rates into  two  parts  by  coagulating ;  that  is,  minute 
fibers  form,  consisting  si  fibrin,  leaving  a  pale  yellowish 
fluid,  called  serum.m  Had  the  blood  not  been  filtered, 
the  corpuscles  and  fibrin  would  have  mingled,  forming 
a  jelly  like  mass,  known  as  clot.  Further,  the  serum  will 
coagulate  if  heated,  dividing  into  hardened  albumen  and 
a  watery  fluid,  called  serosity,  which  contains  the  soluble 
salts  of  the  blood. 

These  several  parts  may  be  expressed  thus :  — 


Blood 


Corpuscles 


Plasma 


colored 


fibrin 
serum 


(  albumen. 

\  serosity = water  and  salts. 


THE    BLOOD   OF  ANIMALS 


303 


FIG.  260.  —  Nucleated  Blood  Cells  of  a  Frog,   x 
250:    a,  colorless  corpuscles. 


If  now  we  examine  the  nutritive  fluid  pi  the  simplest 
animals,  we  find  only  a  watery  fluid  containing  granules. 
In  radiates  and  the 
worms  and  mollusks, 
there  is  a  similar  fluid, 
with  the  addition  of 
a  few  colorless  corpus- 
cles. But  there  is  little 
fibrin,  and,  therefore, 
it  coagulates  feebly 
or  not  at  all.  In  the 
arthropods  and  higher 
mollusks,  the  circulat- 
ing fluid  contains  col- 
orless nucleated  cells, 
and  coagulates.110  In 
vertebrates,  there  are, 

in  addition  to  the  plasma  and  colorless  corpuscles  of 
invertebrates,  red  corpuscles,  to  which  their  blood  owes 
its  peculiar  hue.  In  fishes,  amphibians,  reptiles,  and 
birds,  i.e.,  all  oviparous  vertebrates,  these  red  corpuscles 
are  nucleated ;  but  in  those  of 
mammals,  no  nucleus  has  been 
discovered.111 

All  blood  corpuscles  are  micro- 
scopic.     The    colorless   are    more 
uniform  in  size  than  the  red ;  and 
generally  smaller  (except  in  mam- 
mals), being  about  ^rVo  °f  an  mcn 
in  diameter.     The  red  corpuscles 
are  largest  in  amphibians  (those  of 
Proteus  being  the  extreme,  or  ^o" 
of  an  inch),  next  in  fishes,  then  birds  and  mammals. 
The  smallest  known  are  those  of  the  musk  deer.     In 
mammals,  the  size  agrees  with  the  size  of  the  animal 


FIG.  261.  — Elliptical  Corpuscle 
of  the  Frog,  showing  the  nu- 
cleus as  a  prominence  in  the 
center.  Greatly  magnified. 


304 


COMPARATIVE   ZOOLOGY 


only  within  a  natural  order ;  but  in  birds  the  correspon- 
dence holds  good  throughout  the  class,  the  largest  being 
found  in  the  ostrich,  and  the  smallest  in  the  humming  bird. 
In  man,  they  measure  ^Vo  of  an  inch,  so  that  it  would 
take  40,000  to  cover  the  head  of  a  pin. 

As  to  shape,  the  colorless  corpuscles  are  ordinarily 
globular,  in  all  animals ;  but  they  are  constantly  chang- 


FIG.  262.  —  Comparative  Size  and  Shape  of  the  red  Corpuscles  of  various  Animals. 

ing.  The  form  of  the  red  disks  is  more  permanent, 
although  they  are  soft  and  elastic,  so  that  they  squeeze 
through  very  narrow  passages.  They  are  oval,  circular, 
or  angular,  in  fishes ;  oval  in  reptiles,  birds,  and  the 
camel  tribe  ;  and  circular  in  the  rest  of  mammals.  They 
are  double  convex  when  nucleated,  and  double  concave 
when  circular  and  not  nucleated. 

Blood  is  always  heavier  than  water ;  but  is  thinner  in 
cold-blooded  than   in   warm-blooded   animals,  in  herbi- 


THE   BLOOD   OF  ANIMALS  305 

vores  than  in  carnivores.  The  blood  of  birds,  which  is 
the  hottest  known,  being  104°  F.  which  is  2°-i4°  F. 
higher  than  mammals',  is  richest  in  red  corpuscles.  In 
man,  they  constitute  about  one  half  the  mass  of  blood. 
The  white  globules  are  far  less  numerous  than  the  red ; 
they  are  relatively  more  abundant  in  venous  than  arte- 


FIG.  263. — Capillary  Circulation  in  the  Web  of  a  Frog's  Foot,  X  100:  a,  b,  small  veins; 
d,  capillaries  in  which  the  oval  corpuscles  are  seen  to  follow  one  another  in  single 
series;  c,  pigment  cells  in  the  skin. 

rial  blood,  in  the  sickly  and  ill-fed  than  in  the  healthy 
and  vigorous,  in  the  lower  vertebrates  than  in  birds 
and  mammals.  Their  number  is  subject  to  great  vari- 
ations, increasing  rapidly  after  a  meal,  and  falling  as 
rapidly. 

There  is  less  blood  in  cold-blooded  than  in  warm- 
blooded animals ;  and  the  larger  the  animal,  the  greater 
is  the  proportion  of  blood  to  the  body.     Man  has  about 
DODGE'S  GEN.  ZOOL.  —  20 


306  COMPARATIVE   ZOOLOGY 

a  gallon  and  a  half,  equal  to  one-thirteenth  of  his 
weight.  The  heart  of  the  Greenland  whale  is  a  yard 
in  diameter. 

The  main  Office  of  the  Blood  is  to  supply  nourishment 
to,  and  take  away  waste  matters  from,  all  parts  of  the 
body.  It  is  at  once  purveyor  and  scavenger.  In  its 
circulation,  it  passes,  while  in  the  capillaries,  within  an 
infinitesimal  distance  of  the  various  tissue  cells.  Some  of 
the  plasma,  carrying  the  nutritive  matter  needed,  exudes 
through  the  walls  of  the  capillary  tubes ;  the  tissue  as- 
similates or  makes  like  to  itself  whatever  is  suitable  for 
its  growth  and  repair ;  and  the  lymphatics  take  up  the 
transuded  fluid,  and  return  it  to  the  blood  vessels.  At 
the  same  time,  the  waste  products  of  the  tissues  are  col- 
lected and  brought  through  the  venous  capillaries,  veins, 
and  lymphatics  to  the  excretory  organs.  The  special 
function  of  the  several  constituents  of  the  blood  is  not 
wholly  known.  The  corpuscles  in  the  red  marrow  of 
the  bones  of  some  vertebrates  are  supposed  to  be  the 
source  of  the  red  disks.  The  latter  are  the  carriers  of 
oxygen  which  is  taken  up  by  their  red  matter  (hemo- 
globin) in  the  lungs  and  given  up  to  the  tissues.  The 
same  office  is  performed  by  the  blue  coloring  matter 
(haemocyanin)  in  the  blood  of  certain  invertebrates,  as 
the  squid  and  lobster.  The  carbon  dioxide  is  taken  up 
mainly  by  the  plasma. 

Like  the  solid  tissues,  the  blood,  which  is  in  reality  a 
liquid  tissue,  is  subject  to  waste  and  renewal,  to  growth 
and  decay.  The  loss  is  repaired  from  the  products  of 
digestion,  carried  to  the  blood  by  the  lacteals,  or  ab- 
sorbed directly  by  the  capillaries  of  the  digestive  tract. 
The  white  corpuscles  probably  are  prepared  in  many 
parts  of  the  body,  especially  the  liver,  spleen,  and  lym- 
phatic glands.  In  the  lower  organisms,  the  nutritive 
food  is  prepared  by  contact  with  the  tissues,  without 


THE   BLOOD  OF  ANIMALS  307 

passing  through  special  organs.  Lymph  differs  from 
blood  chiefly  in  containing  less  albumen  and  fibrin,  and 
no  red  disks.  Chyle  is  lymph  loaded  with  fat  globules, 
and  is  found  in  the  lacteals  and  vessels  connected  with 
them  during  the  absorption  of  food  containing  fat. 


CHAPTER    XVI* 


THE  CIRCULATION  OF  THE  BLOOD 


The  Blood  is  kept   in   continual   motion 
nourish  and  purify  the  body  and  itself. 

means 
,    .          brings 


in  order  to 
For  as  life 
work,  and  work 
waste,    there    is 
constant  need  of   fresh 
material  to  make  good 
the  loss  throughout  the 
system,  and  of   the  re- 
moval of  matter  which 
is  no  longer  fit  for  use. 

In  the  very  lowest  ani- 
mals, where  all  parts  of 
the  structure  are  equally 
capable  of  absorbing  the 
digested  food  and  are  in 
contact  with  it,  there  is 
no  occasion  for  any  cir- 
culation, although  even 
in  them  the  digested 
food  is  not  allowed  to 
stagnate.  But  in  propor- 
tion as  the  power  of  ab- 
sorption is  confined  to 
certain  parts,  the  more  is 
the  need  and  the  greater  the  complexity  of  an  apparatus 
for  conveying  the  nutritive  fluid  to  the  various  tissues. 

*  See  Appendix. 


FIG.  264.  —  Venous  Valves.     They  usually  oc- 
cur in  pairs,  as  represented. 


THE   CIRCULATION   OF   THE   BLOOD 


309 


In  nearly  all  animals,  the  nutritive  fluid  is  conveyed 
to  the  various  parts  of  the  body  by  a  system  of  tubes, 
called  blood  vessels.  The  higher  forms  have  two  sets  — 
arteries  and  veins,  in  which  the  blood  moves  in  opposite 
directions,  the  former  carrying  blood  from  a  central 
reservoir  or  heart,  the  latter  taking  it  to  the  heart.  In 
the  vertebrates  the  walls  of  these 
tubes  are  made  of  three  coats, 
or  layers,  of  tissue,  the  arteries 
being  elastic,  like  rubber,  and 
many  of  .  the  veins  being  fur- 
nished with  valves.112  The  great 
artery  coming  out  of  the  heart 
is  called  aorta,  and  the  grand 
venous  trunk,  entering  the  heart 
on  the  opposite  side,  is  called 
vena  cava.  Both  sets  divide  and 
subdivide  until  their  branches 
are  finer  than  hairs;  and  joining 
these  finest  arteries  and  finest 
veins  are  intermediate  micro- 
scopic tubes,  called  capillaries 
(in  man  about  3^0  of  an  inch  in 
diameter).113  In  these  only,  so 
thin  and  delicate  are  their  walls, 
does  the  blood  come  in  contact 
with  the  tissues  or  the  air. 

In  those  vertebrates  which  have  lungs  there  are  two 
sets  of  capillaries,  since  there  are  two  circulations  —  the 
systemic,  from  the  heart  around  the  system  to  the  heart 
again,  and  the  pulmonary,  from  the  heart  through  the 
respiratory  organ  back  to  the  heart.  This  double 
course  may  be  illustrated  by  the  figure  8.  In  gill- 
bearing  animals  there  are  capillaries  in  the  gills,  but 
not  a  double  circulation. 


FIG.  265.  —  Relation  of  artery,  a, 
vein,  6,  and  capillaries,  c,  as  seen 
in  the  muscles  of  a  Dog. 


COMPARATIVE  ZOOLOGY 


There  is  no  true  system  of  blood  vessels  below  the 
echinoderms.  The  simplest  provision  for  the  distribu- 
tion of  the  products  of  digestion  is  shown  by  the  jelly- 
fish, whose  stomach  sends  off  radiating  tubes  (Fig.  21), 
through  which  the  digested  food  passes  directly  to  the 
various  parts  of  the  body  instead  of  being  carried  by 
the  agency  of  a  circulating  medium  —  viz.,  the  blood. 

The  First  Approach  to  a  Circulatory  System  is  made  by 
the  starfish  and  the  sea  urchin.  A  vein  runs  along  the 
whole  length  of  the  alimentary  tube,  to 
absorb  the  chyle,  and  forms  a  circle 
around  each  end  of  the  tube.  These 
circular  vessels  send  off  branches  to 
various  parts  of  the  body ;  but  as 
they  are  not  connected  by  a  network 
of  capillaries,  there  can  be  no  circuit 
(Fig.  237). 

A  higher  type  is  exhibited  by  the 
insects.  If  we  examine  the  back  of 
any  thin-skinned  caterpillar,  a  long 
pulsating  tube  is  seen  running  beneath 
the  skin  from  one  end  of  the  body  to 
the  other.  This  dorsal  vessel,  or  heart, 
as  it  is  called,  is  open  at  both  ends,  and 
divided  by  valves  into  compartments, 
permitting  the  blood  to  go  forward, 
but  not  backward.  Each  compartment 
communicates  by  a  pair  of  slits,  guarded 
by  valves,  with  the  body  cavity,  so  that 
fluids  may  enter,  but  cannot  escape. 
"  Circulation  "  is  very  simple.  We  have 
seen  that  the  chyle  exudes  through  the 
walls  of  the  alimentary  canal  directly 
into  the  cavity  of  the  abdomen,  where  it  mingles  with 
the  blood  already  there.  This  mixed  fluid  is  drawn 


FIG.  266.  —  Part  of  the 
Dorsal  Vessel,  or 
Heart,  of  a  Cock- 
chafer bisected :  a,  b, 
muscular  walls;  rf, 
valves  between  the 
compartments;  c, 
valve  defending  one 
of  the  orifices  com- 
municating with  the 
general  cavity  of  the 
abdomen. 


THE   CIRCULATION   OF   THE    BLOOD  311 

into  the  dorsal  tube  through  the  valvular  openings  as  it 
expands ;  and  upon  its  contraction,  all  the  side  valves 
are  closed,  and  the  fluid  is  forced  toward  the  head. 
Passing  out  at  the  front  opening,  it  is  again  diffused 
among  and  between  the  tissues  of  the  body.  The 
blood,  therefore,  does  not  describe  a  circle  in  definite 
channels  so  as  to  return  constantly  to  its  point  of 
departure. 

Many  worms  (as  the  earthworm)  have  a  pulsating 
tube  extending  from  tail  to  head  above  the  alimentary 
canal,  a  similar  tube  on  the  ventral  side  through  which 


FIG.  267.  —Circulation  in  a  Lobster:  a,  heart;  3,  artery  for  the  eyes;  c,  artery  for  an- 
tennae; d,  hepatic  artery;  e,  superior  abdominal  artery:  f,  sternal  artery;  g,  venous 
sinuses  transmitting  blood  from  the  body  to  the  branchiae,  h,  whence  it  returns  to  the 
heart  by  the  branchio-cardiac  vessels,  i. 

the  blood  returns,  and  cross  tubes  in  every  segment 
(Fig.  52).  In  the  lobster  and  crab,  spider  and  scorpion, 
the  dorsal  tube  sends  off  a  system  of  arteries  (not  found 
in  insects) ;  but  the  blood,  as  it  leaves  these  tubes, 
escapes  into  the  general  cavity,  as  in  t>ther  Arthropoda. 
The  lobster  and  crab,  however,  show  a  great  advance 
in  the  concentration  of  the  propelling  power  into  a 
short  muscular  sac. 

A  third  development  of  the  circulatory  system  is 
furnished  by  the  mollusks.  Comparatively  sluggish, 
they  need  a  powerful  force  pump  in  the  form  of  a  com- 
pact heart.  In  the  oyster  and  snail  (Figs.  242,  243), 
we  find  such  an  organ  having  two  cavities  —  an  auricle 


312 


COMPARATIVE   ZOOLOGY 


and  a  ventricle,  one  for  receiving,  and  the  other  for 
distributing,  the  blood.  The  auricle  injects  the  blood 
into  the  ventricle,  which  propels  it  by  the  arteries  to 
the  various  organs.  Thence  it  passes  not  immediately 

to  the  veins,  as  in  higher  ani- 
mals, but  into  the  spaces  around 
the  alimentary  canal.  A  part 
of  this  is  carried  by  vessels  to 
the  gills  or  lung,  and  then  re- 
turned with  the  unpurified  por- 
tion to  the  auricle.  The  whole 
of  the  blood,  therefore,  does 
not  make  a  complete  circuit. 
The  clam  has  a  similar  heart, 
but  with  two  auricles. 

A  still  higher  form  is  seen 
in  the  cuttlefish,  the  highest  of 
the  invertebrates.  This  animal 
has  a  central  heart,  with  a 
ventricle  and  two  auricles, 
and,  in  addition,  the  veins 
which  collect  the  blood  from 
the  system  to  send  it  back  to 
the  heart  by  the  way  of  the 
gills  are  furnished  with  two 
branchial  hearts,  which  accel- 
erate the  circulation  through 
those  organs.  Many  of  the 
arteries  and  veins  are  joined 
by  capillaries,  but  not  all  ; 
so  that  in  no  invertebrate 

animai  is  the  bi°°d  returned 

ricle;    e,  venous  sinus;  /,  portal      to     ^Q    heart    by    a     COntinUOUS 

vein;  g,  intestine;    h,  vena  cava;  J 

f,  branchial  vessels;  k,  dorsal  ar-      closed      System      of      blood      VCS- 

tery,    or    aorta;     I,    kidneys;     m,  •> 

dorsal   artery.  S61S. 


h~ 


FIG  268.—  circulating  Apparatus  in 


THE   CIRCULATION    OF   THE   BLOOD  313 

As  a  rule,  in  all  animals  having  any  circulation  at  all, 
the  current  always  takes  one  direction.  This  is  gener- 
ally necessitated  by  valves.  But  a  curious  exception  is 
presented  by  the  ascidians,  whose  tubular  heart  is  valve- 
less,  and  the  contractions  occur  alternately  at  one  end 
and  then  the  other ;  so  that  the  blood  oscillates  to  and 
fro,  and  a  given  vessel  is  at  one  time  a  vein  and  at  an- 
other an  artery.  In  this  respect  it  resembles  the  foetal 
heart  of  higher  animals  (Fig.  364). 

In  vertebrates  only  is  the  circulating  current  strictly 
confined  to  the  blood  vessels ;  in  no  case  does  it  escape 
into  the  general  cavity 
of  the  body.  In  other 
respects,  there  is  no  great 
advance  in  the  apparatus 
of  the  lowest  vertebrates 
over  that  of  the  highest 
mollusks.  A  fish's  heart 
has,  like  that  of  an  oyster, 
two  cavities,  but  its  posi- 
tion is  reversed.  Instead 
of  driving  arterial  blood 
over  the  body,  it  receives 
the  returning,  or  venous,  FlG  2&9  "_ DiagraVof  a  Singie  Heart!  <f, 

blood,  and    Sends    it    tO    the        auricle'    '>  venficle;    c,  veins  leading  to 

auricle;  a,  aorta,  or  mam  artery. 

gills.    Recollected  from  the 

gills,  the  blood  is  passed  into  a  large  artery,  or  aorta, 
along  the  back,  which  distributes  it  by  a  complex  sys- 
tem of  capillaries  among  the  tissues.  These  capillaries 
unite  with  the  ends  of  the  veins  which  pass  the  blood 
into  the  auricle114  (Figs.  268,  272). 

In  amphibians  and  in  reptiles  generally  (as  frogs, 
snakes,  lizards,  and  turtles),  the  heart  has  three  cavities 
—  two  auricles  and  one  ventricle.  The  venous  blood 
from  the  body  is  received  into  the  right  auricle,  and  the 


314 


COMPARATIVE   ZOOLOGY 


purified  blood  from  the  lungs  into  the  left.  Both  throw 
their  contents  into  the  ventricle,  which  pumps  the  mixed 
blood  in  two  directions  —  partly  to  the  lungs,  and  partly 
around  the  system  (Fig.  273).  Circulation  is,  therefore, 
incomplete,  since  the  whole  current  does  not  pass 
through  the  lungs,  and  three  kinds  of  blood  are  found 
in  the  body  —  arterial,  venous,  and  mixed.  In  many 
animals,  however,  arrangements  exist  which  nearly 
separate  the  venous  from  the  arterial  blood. 

The  ventricle  of  reptiles  is  partially  divided  by  a  par- 
tition. In  the  crocodile,  the  division  is  complete,  so  that 
there  are  really  four  cavities  —  two  auricles,  and  two 
ventricles.  But  both  ventricles  send  off  aortas  which 
cross  one  another,  and  at  that  point  a  small  aperture 
brings  the  two  into  communication.  The  venous  and 
arterial  currents  are,  therefore,  mixed,  but  not  within 
the  heart,  as  in  other  reptiles,  nor  so  extensively.  In 

the  structure  of  the  heart, 
as  well  as  in  that  of  the 
gizzard,  crocodiles  approach 
the  birds. 

The  Highest  Form  of  the 
Circulating  System  is  pos- 
sessed by  the  warm-blooded 
vertebrates,  birds  and 
mammals.  Not  a  drop  of 
blood  can  make  the  circuit 
of  the  body  without  passing 

throush  the  lunss'  the  dr- 

culation    tO    and   from    thoSC 

Organs    being    aS    perfect    aS 

,  ,.         .,  f  .    , 

the  distribution  of  arterial 
blood.  The  heart  consists  of  four  cavities  —  a  right 
auricle  and  ventricle,  and  a  left  auricle  and  ventricle. 
In  other  words,  it  is  a  hollow  muscle  divided  internally 


separated  than  in  higher  animals:  E, 
right  ventricle;  L,  left  ventricle;  D, 
right  auricle;  F,  pulmonary  artery; 
K,  left  auricle;  A,  aorta. 


THE   CIRCULATION   OF   THE   BLOOD 


315 


by  a  vertical  partition  into  two  distinct  chambers,  each 
of  which  is  again  divided  by  a  valve  into  an  auricle 
and  a  ventricle.  The  work  of  the  right  auricle  and 
ventricle  is  to  receive  the  blood  from  the  veins,  and 
send  it  to  the  lungs ;  while  the  other  two  receive 
the  blood  from  the  lungs,  and  propel  it  over  the  body. 
The  left  ventricle  has  more  work  to  do  than  any  of 


/     g 


FIG.  271.  — Theoretical  Section  of  the 
Human  Heart:  a,  right  ventricle; 
b,  inferior  vena  cava;  c,  tricuspid 
valve;  d,  right  auricle;  e,  pulmonary 
veins;  _/,  superior  vena  cava;  g,  pul- 
monary arteries;  h,  aorta;  k,  left 
auricle;  /,  mitral  valve;  m,  left  ven- 
tricle; n,  septum. 


FIG.  272.  —  Plan  of  Circula- 
tion in  Fishes:  a,  auricle; 
b,  ventricle;  c,  branchial 
artery;  e,  branchial  veins, 
bringing  blood  from  the 
gills,  d,  and  uniting  in  the 
aorta,y/  g,  vena  cava. 


the  other  parts  of  the  heart.  The  two  auricles  con- 
tract at  the  same  instant;  so  also  do  the  ventricles. 
The  course  of  the  current  in  birds  and  mammals  is  as 
follows :  the  venous  blood  brought  from  the  system  is 
discharged  by  two  or  three  large  trunks 115  into  the  right 
auricle,  which  immediately  forces  it  past  a  valve 116  into 
the  right  ventricle.  The  ventricle  then  contracts,  and 
the  blood  is  forced  through  the  pulmonary  artery  past 
its  semilunar  valves  into  the  lungs,  where  it  is  changed 


COMPARATIVE  ZOOLOGY 


from  venous  to  arterial,  returning  by  the  pulmonary 
veins  to  the  left  auricle.  This  sends  it  past  the  mitral 
valves  into  the  left  ventricle,  which  drives  it  past  the 
semilunar  valves  into  the  aorta,  and  thence,  by  its  rami- 
fying arteries  and  capillaries,  into  all  parts  of  the  body 
except  the  lungs.  From  the  systemic  capillaries,  the 
blood,  now  changed  from  arterial  to  venous,  is  gathered 
by  the  veins,  and  conveyed  back  to  the  heart. 

The  Rate  of  the  Blood  Current  generally  increases  with 
the  activity  of  the  animal,  being  most  rapid  in  birds.117 

In  insects,  however, 
it  is  comparatively 
slow ;  but  this  is 
because  the  air  is 
taken  to  the  blood 
—  the  whole  body 
being  bathed  in  air, 
so  that  the  blood  has 
no  need  to  hasten 
to  a  special  organ. 
However,  activity 
nearly  doubles  the 
rate  of  pulsation  in 
a  bee.  The  motion 
in  the  arteries  is 
several  times  faster 
than  in  the  veins, 
but  diminishes  as 
the  distance  from  the  heart  increases.  In  the  carotid 
of  the  horse,  the  blood  moves  12^  inches  per  second ;  in 
that  of  man,  16 ;  in  the  capillaries  of  man,  I  to  2  inches 
per  minute  ;  in  those  of  a  frog,  i. 

The  Cause  of  the  Blood  Current  may  be  cilia,  or  the 
contractions  of  the  body,  or  pulsating  tubes  or  hearts. 
In  the  higher  animals,  the  impulse  of  the  heart  is  not  the 


FIG  273.  —A,  Plan  of  Circulation  in  Amphibia  and 
Reptiles;  B,  Plan  of  Circulation  in  Birds  and 
Mammals:  a,  right  auricle  receiving  venous  blood 
from  the  system;  b,  left  auricle  receiving  arterial 
blood  from  the  lungs;  c,  c',  ventricles;  d,  e,ft 
systemic  artery,  vein,  and  capillaries;  g,  pulmon- 
ary artery;  h,  k,  vein  and  capillaries. 


THE   CIRCULATION   OF   THE   BLOOD  317 

sole  means  :  it  is  aided  by  the  contractions  of  the  elastic 
walls  of  the  arteries  themselves,  the  movements  of  the 
chest  in  respiration,  and  the  attraction  of  the  tissues  for 
the  arterial  blood  in  the  capillaries.  In  the  chick,  the 
blood  mores  before  the  heart  begins  to  beat ;  and  if  the 
heart  of 'an  animal  be  suddenly  taken  out,  the  motion  in 
the  capillaries  will  continue  as  before.  It  has  been 
estimated  that  the  force  which  the  human  heart  expends 
in  twenty-four  hours  is  about  equivalent  to  lifting  217 
tons  one  foot. 


CHAPTER    XVII* 

HOW  ANIMALS   BREATHE 

Arterial  Blood,  in  passing  through  the  system,  both 
loses  and  gains  certain  substances.  It  loses  constructive 
material  and  oxygen  to  the  tissues.  These  losses  are 
made  good  from  the  digestive  tract  and  breathing  organ. 
It  gains  also  certain  waste  materials  from  the  tissues, 
which  must  be  got  rid  of.  Of  these  waste  products,  one, 
carbon  dioxide,  is  gaseous,  and  is  passed  off  from  the 
same  organ  as  that  where  the  oxygen  is  taken  in.  This 
exchange  of  gases  between  the  animal  and  its  surround- 
ings is  called  respiration. 

The  First  Object  of  Respiration  is  to  convert  venous 
into  arterial  blood.  It  is  done  by  bringing  it  to  the  sur- 
face, so  that  carbon  dioxide  may  be  exhaled  and  oxygen 
absorbed.  The  apparatus  for  this  purpose  is  analogous 
to  the  one  used  for  circulation.  In  the  lowest  animals, 
the  two  are  combined.  But  in  the  highest,  each  is 
essentially  a  pump,  distributing  a  fluid  (in  one  case  air, 
in  the  other  blood)  through  a  series  of  tubes  to  a  system 
of  cells  or  capillaries.  They  are  also  closely  related  to 
each  other :  the  more  perfect  the  circulation,  the  more 
careful  the  provision  made  for  respiration, 

Respiration  is  performed  either  in  air  or  in  water.  So 
that  all  animals  may  be  classed  as  air  breathers  or  water 
breathers.  The  latter  are,  of  course,  aquatic,  and  seek 
the  air  which  is,  dissolved  in  the  water.  Land  snails, 
myriapods,  spiders,  insects,  reptiles,  birds,  and  mam- 

*  See  Appendix. 


HOW  ANIMALS    BREATHE 


319 


mals  breathe  air  directly ;  the  rest,  with  few  exceptions, 
receive  it  through  the  medium  of  water.  In  the  former 
case,  the  organ  is  internal ;  in  the  latter,  it  is  more  or  less 
on  the  outside.  But  however  varied  the  organs  —  tubes, 
gills,  or  lungs  —  they  are  all  constructed 
on  the  same  principle  —  a  thin  membrane 
separating  the  blood  from  the  atmosphere. 

(i)  Protozoa,  Sponges,  and  Polyps  have  no 
separate  respiratory  apparatus,  but  absorb 
air,  as  well  as  food,  from  the  currents  of 
water  passing  through  them  or  bathing 
the  surface  of  their  bodies. 

In  the  starfish,  sea  urchin,  and  the 
like,  we  find  the  first  distinct  respiratory 
organs,  although  none  are  exclusively 
devoted  to  respiration.  There  are  two 
sets  of  canals  —  one  carrying  the  nutrient 
fluid,  and  the  other,  radiating  from  a  ring 
around  the  mouth,  distributing  aerated 
water,  used  for  locomotion  as  well  as 
respiration.  This  may  be  called  the 
"water-pipe  system."  Besides  this,  there 
are  sometimes  numerous  gill-like  fringes, 
which  cover  the  surface  of  the  body  and  FIG.  274.— Lobworm 

v     1.1  '  i    •  '4.' 

probably  aid  in  respiration. 

Freshwater  worms,  like  the  leech  and 
earthworm,  breathe  by  the  skin.  The 
body  is  -always  covered  by  a  viscid  fluid, 
which  has  the  property  of  absorbing  air. 
The  air  is  therefore  brought  into  immediate  contact 
with  the  soft  skin,  underneath  which  lies  a  dense  net- 
work of  blood  vessels. 

But  most  water-breathing  animals  have  gills.  The 
simplest  form  is  seen  in  marine  worms  :  delicate  veins 
projecting  through  the  skin  make  a  series  of  arborescent 


(A  renicola  piscato- 
rum)t  a  dorsibran- 
chiate,  showing  the 
tufts  of  capillaries, 
or  external  gills. 
The  large  head  is 
without  eyes  or 
jaws. 


320 


COMPARATIVE   ZOOLOGY 


tufts  along  the  side  of  the  body ;  as  these  float  in  the 
water,  the  blood  is  purified.118  Bivalve  mollusks  have 
four  flat  gills,  consisting  of  delicate  membranes  filled 
with  blood  vessels  and  covered  with  cilia.  In  the 
oyster,  these  ribbonlike  folds  are  exposed  to  the  water 
when  the  shell  opens ;  but  in  the  clam,  the  mantle 
incloses  them,  forming  a  tube,  called  siphon,  through 
which  the  water  is  driven  by  the  cilia.  The  aquatic 

gastropods  (univalves)  have 
either  tufts,  like  the  worms, 
or  comblike  ciliated  gills  in 
a  cavity  behind  the  head,  to 
which  the  water  is  admitted 
through  an  opening.  In 
others  the  breathing  organ 
is  the  vascular  lining  of  this 
cavity.  The  cuttlefish  has 
flat  gills  covered  by  the  man- 
tle; but  the  water  is  drawn 
in  by  muscular  contractions 
of  the  mantle  instead  of  by 
cilia.  The  end  of  the  siphon 
through  which  it  is  ejected  is 
called  the  funnel.  The  gills 
of  lobsters  and  crabs  are 

placed  in  cavities  covered  by  the  sides  of  the  shell 
(carapace) ;  and  the  water  is  brought  in  from  behind  by 
the  action  of  a  scoop-shaped  process  attached  to  one  of 
the  jaws,  which  constantly  bails  the  water  out  at  the 
front. 

The  perfection  of  apparatus  for  aquatic  respiration  is 
seen  in  fishes.  The  gills  are  comblike  fringes  supported 
on  four  or  five  bony  or  cartilaginous  arches,  and  contain 
myriads  of  microscopic  capillaries,  the  object  being  to 
expose  the  venous  blood  in  a  state  of  minute  subdivision 


FIG  275.  —  Diagrammatic  Section  of  a 
Lamellibranch  (Anodonta):  a,  lobes 
of  mantle;  b,  gills,  showing  transverse 
partitions;  c,  ventricle  of  heart;  d, 
auricles;  e,  pericardium;  f,  g,  kid- 
neys; h,  venous  sinus;  k,  foot;  A, 
branchial,  or  pallial,  chamber;  B, 
epibranchial  chamber. 


HOW   ANIMALS    BREATHE 


321 


FIG.  276!  —  Spiracle  of  an  Insect,  x  75. 


to  streams  of  water.     The  gills  are  always  covered.     In 

bony  fishes  they  are  attached  to  the  hinder  side  of  bony 

arches,  all  covered  by 

a    flap    of    the    skin, 

supported    by    bones 

(the    gill    cover,     or 

operculum\    and    the 

water    escapes    from 

the    opening    left    at 

its  hinder  edge.     In 

sharks,   the  gills  are 

placed     in     pouches 

which  open  separately 

(Figs.  122,360).    The 

act     of      "  breathing 

water"    resembles 

swallowing,  only  the  water  passes  over  the  surface  of 

the  gills  instead  of  entering  the  gullet. 

(2)  Air  Breathers  have  trachea,  or  lungs.     The  former 

consist  of  two  principal  tubes,  which  pass  from  one  end 

of  the  body  to  the  other,  opening  on  the  surface  by 
apertures,  called  spiracles,  resembling  a 
row  of  buttonholes  along  the  sides  of 
the  thorax  and  abdomen,  and  ramifying 
through  the  smallest  and  most  delicate 
organs,  so  that  the  air  rriay  follow  the 
blood  wherever  it  circulates..  To  keep 
the  pipes  ever  open,  and  at  the  same 
.  time  leave  them  flexible,  they  are  pro- 

FG.  277.  —  Tracheal 

inside  with  an  elastic  spiral  thread, 
a    droplight. 

Respiration  is  performed  by  the  move- 
ments of  the  abdomen,  as  may  be  seen  in  the  bee  when 
at  rest.  This  "  air-pipe  system,"  as  it  may  be  termed, 
is  best  developed  in  insects. 


Tube  of  an  Insect, 
highly    magnified, 

showing     elastic    like    the    rubber    tube    of 

spiral    thread. 


DODGE'S  GEN.  ZOOL.  —  21 


322 


COMPARATIVE   ZOOLOGY 


The  "nerves"  of  an  insect's  wing  consist  of  a  tube 
within  a  tube,  the  inner  one  is  a  trachea  carrying  air, 
and  the  outer  one,  sheathing  it,  is  a  blood  vessel.  So 


FIG.  278.  —Ideal  Section  of  a  Bee:    a,  alimentary  canal;    //,  dorsal  vessel;    t,  trachea: 
n,  nervous  cord. 

perfect  is  the  aeration  of  the  whole  body,  from  brain  to 
feet,  the  blood  is  oxygenated  at  the  moment  when,  and 
on  the  spot  where,  it  is  carbonized ;  only  one  kind  of 
fluid  is,  therefore,  circulating  —  arterial.  It  is  difficult 
to  drown  an  insect,  as  the  water 
can  not  enter  the  pores ;  but  if  a 
drop  of  oil  be  applied  to  the  abdo- 
men, it  falls  dead  at  once,  being 
suffocated.  The  largest  spiracle  is 
usually  found  on  the  thorax,  as 
under  the  wing  of  a  moth ;  such 
may  be  strangled  by  pinching  the 
thorax. 

In  millepedes  and  centipedes,  the 
spiracles   open  into  little  sacs  con- 
nected together  by  tubes;  in  spiders  and  scorpions,  the 


FIG.  279.  —  Section  of  in- 
jected Lung  (highly  mag- 
nified):  a,  a,  free  edges 
of  alveoli;  c,  c,  partitions 
between  neighboring  al- 
veoli; b,  small  arterial 
branch  giving  off  capil- 
laries to  the  alveoli. 


HOW  ANIMALS   BREATHE 


323 


spiracles,  usually  four  in  number,  are  the  mouths  of 
sacs  without  the  tubes,  and  the  interior  of  the  sac  is 
gathered  into  folds.  Land  snails  have  one  spiracle, 
or  aperture,  on  the  right  side  of  the  neck,  leading  to 
a  large  cavity,  or  sac,  lined  with  fine  blood  vessels. 
These  sacs  represent  the  primitive  idea  of  a  lung,  which 
is  but  an  infolding  of  the  skin,  divided  up  into  cells,  and 
covered  with  capillary  veins.119 


FIG.  280.  —  Part  of  a  Transverse  Section  of  a  Pig's  Bronchial  Twig,  x  240:  a,  outer 
fibrous  layer;  bt  muscular  layer;  c,  inner  fibrous  layer;  d,  epithelial  layer  with  cilia; 
f,  one  of  the  neighboring  alveoli. 

Like  the  alimentary  canal,  the  lungs  of  an  animal  are 
really  an  inflected  portion  of  the  outer  surface ;  so  that 
breathing  by  the  skin  and  breathing  by  lungs  are  one  in 
principle.  Indeed,  in  many  animals,  especially  frogs, 
respiration  is  carried  on  by  both  lungs  and  skin. 

In  the  course  of  embryonic  development  the  lungs 
of  vertebrates  are  derived  from  the  front  part  of 
the  alimentary  canal.  In  some  fishes,  air  is  swal- 
lowed, which  passes  the  whole  length  of  the  diges- 
tive tract,  and  is  expelled  from  the  anus.  Here  the 
whole  canal  serves  for  respiration.  In  reptiles,  birds, 


324 


COMPARATIVE   ZOOLOGY 


and  mammals  the  hinder  part  of  the  intestine  develops 
an  outgrowth  (the  allantois}  during  embryo  life  which 
serves  as  the  embryo's  breathing  organ 
(Figs.  365,  366). 

All  vertebrates  have  two  kinds  of  re- 
spiratory organs  in  the  course  of  their 
life.  Fishes  have  gills  ;  their  lung  (the  air 
bladder)  rarely  serves  as  a  functional  re- 
spiratory organ,  and  is  sometimes  wanting. 
Amphibians  have  gills  while  in  the  larval 
state.  Some  keep  them  throughout  life ; 
but  all  develop  functional  lungs,  and  also 
breathe  by  means  of  the  skin. 

In  the  remaining  vertebrates,  the  allan- 
tois  is  the  breathing  organ  of  the  embryo, 
and  the  lung  is  the  breathing  organ  of 
the  adult.  The  skin  is  of  small  or  no 
importance  in  respiration. 

The  lungs  of  vertebrates  are  elastic, 
membranous  sacs,  divided  more  or  less 
into  cavities  (the  air  cells]  to  increase  the 
surface.  Upon  the  walls  of  the  air  cells 
are  spread  the  capillary  blood  vessels. 
The  smaller  the  cells,  the  greater  the 
extent  of  surface  upon  which  the  blood 
lS  a8s'nake"n^  *s  exposed  to  the  influence  of  the  air,  and, 
trachea;  t,  its  therefore,  the  more  active  the  respiration 

bifurcation;       c, 

pulmonary    ar-  and  the  purer  the  blood.     The  lungs  are 
naj  veinTThe   relatively  largest  in  reptiles,  and  smallest 


lung,  B,  is  rudi- 
mentary. 


in  mammals.  But  in  the  cold-blooded 
amphibians  and  reptiles,  the  air  cells  are 
few  and  large ;  in  the  warm-blooded  birds  and  mammals, 
they  are  exceedingly  numerous  and  minute.120  Respira- 
tion is  most  perfect  in  birds ;  they  require,  relatively 
to  their  weights,  more  air  than  mammals  or  reptiles,  and 


HOW  ANIMALS   BREATHE 


325 


most  quickly  die  for  lack  of  it.  In  birds,  respiration  is 
not  confined  to  the  lungs  ;  but,  as  in  insects,  extends 
through  a  great  part  of  the  body.  Air  sacs  connected 
with  the  lungs  exist  in  the  abdomen  and  under  the 
skin  of  the  neck,  wings,  and  legs.  Even  the  bones 
are  hollow  for  this  purpose ;  so  that  if  the  windpipe  be 


FIG.  282. —  Lungs  of  a  Frog;  a, 
hyoid  apparatus;  b,  cartilaginous 
ring  at  root  of  the  lungs;  c,  pul- 
monary vessels;  d,  pulmonary 
sacs,  having  this  peculiarity  com- 
mon to  all  cold-blooded  air  breath- 
ers, that  the  trachea  does  not 
divide  into  bronchial  branches,  but 
terminates  abruptly  by  orifices 
which  open  at  once  into  the  gen- 
eral cavity.  A  cartilaginous  net- 
work divides  the  space  into  little 
sacs,  on  the  walls  of  which  the 
capillaries  are  spread. 


FIG.  283.  —  Distribution  of  Air  Tubes  in  Mam- 
malian Lungs:  a,  larynx;  bt  trachea;  c,  d,  left 
and  right  bronchial  tubes:  e,f.  g,  the  ramifica- 
tions. In  Man  the  subdivision  continues  until 
the  ultimate  tubes  are  one  twenty-fifth  of  an 
inch  in  diameter.  Each  lobule  represents  in 
miniature  the  structure  of  the  entire  lung  of  a 
Frog. 


tied,  and  an  opening  be  made  in  the  wing  bone,  the  bird 
will  continue  to  respire.  The  right  lung  is  usually  the 
larger ;  in  some  snakes,  the  left  is  wanting  entirely. 
In  most  vertebrates,  lungs  are  freely  suspended ;  in 
birds,  they  are  fastened  to  the  back. 

The  lungs  communicate  with  the  atmosphere  by 
means  of  the  trachea,  or  windpipe,  formed  of  a  series  of 
cartilaginous  rings,  which  keep  it  constantly  open.  It 


326 


COMPARATIVE   ZOOLOGY 


begins  in  the  back  part  of  the  mouth,  opening  into  the 
pharynx  by  a  slit,  called  the  glottis,  which,  in  mammals, 
is  protected  by  the  valvelike  epiglottis.  The  trachea 
passes  along  the  neck  in  front  of  the  esophagus,  and 
divides  into  two  branches,  or  bronchi,  one  for  each  lu-ng. 
In  birds  and  mammals,  the  bronchial  tubes,  after  enter- 
ing the  lungs,  subdivide  again  into  minute  ramifications. 
Vertebrates  are  the  only  animals  that  breathe  through 
the  mouth  or  nostrils.  Frogs,  having  no  ribs,  and  tur- 
tles, whose  ribs  are  soldered  together  into  a  shield, 

are  compelled  to 
swallow  the  air. 
Snakes,  lizards, 
and  crocodiles 
draw  it  into  the 
lungs  by  the  play 
of  the  ribs.121 
Birds,  unlike  other 
animals,  do  not 
inhale  the  air  by 
an  active  effort ; 
for  that  is  done  by 
the  springing  back 
of  the  breastbone  and  ribs  to  their  natural  position. 
To  expel  the  air,  the  breastbone  is  drawn  down  toward 
the  backbone  by  muscles,  which  movement  compresses 
the  lungs. 

Mammals  alone  have  a  perfect  thorax  —  i.e.,  a  closed 
cavity  for  the  heart  and  lungs,  with  movable  walls 
(breastbone  and  ribs)  and  the  diaphragm,  or  muscular 
partition,  separating  it  from  the  abdomen.122  Inspira- 
tion (or  filling  the  lungs)  and  expiration  (or  emptying 
the  lungs)  are  both  accomplished  by  muscular  exertion  ; 
the  former,  by  raising  the  ribs  and  lowering  the  di- 
aphragm, thus  enlarging  the  capacity  of  the  chest,  in 


FIG.  284.  —Skeleton  of  a  Frog. 


HOW  ANIMALS   BREATHE 


327 


consequence  of  which  the  air  rushes  in  to  prevent  a 
vacuum ;  the  latter,  by  the  ascent  of  the  diaphragm  and 
the  descent  of  the  ribs. 

As  a  rule,  the  more  active  and  more  muscular  an 
animal,  the  greater  the  demand  for  oxygen.  Thus, 
warm-blooded  animals  live  fast,  and  their  rapidly  decay- 
ing tissues  call  for  rapid 
respiration ;  while  in  the 
cold-blooded  creatures  the 
waste  is  comparatively 
slow.  Respiration  is  most 
active  in  birds,  and  least  in 
water-breathing  animals. 
The  sluggish  toad  respires 
more  slowly  than  the  busy 
bee,  the  mollusk  more 
slowly  than  the  fish. 
But  respirations,  like 
beats  of  the  heart,  are 
fewer  in  large  mammals 
than  in  small  ones.  An 
average  man  inhales  about 
300-400  cubic  feet  of  air 
per  day  of  rest,  and  much 

* 
more  When  at  WOrk. 

.  ,  i          r 

Another     reSUlt     OI    reS- 

piration,  besides  the  puri- 
fication   of   the  blood,  is 

the  production  of  heat.  The  chemical  combination  of 
the  oxygen  in  the  air  with  the  carbon  in  the  tissues 
is  a  true  combustion ;  and,  therefore,  the  more  active 
the  animal  and  its  breathing,  the  higher  its  tempera- 
ture. Birds  and  mammals  have  a  constant  temperature, 
which  is  usually  higher  than  that  of  the  atmosphere 
(108°  and  100°  F.  respectively).  They  are  therefore 


/ 


FIG.  285.  — Human  Thorax:  a,  vertebral  col- 
umn; b,  b' ,  ribs,  the  lower  ones  false;  c, 
clavicle;  e,  intercostal  muscles,  removed  on 
the  left  side  to  show  the  diaphragm,  d ;  f, 
pillars  of  the  diaphragm  attached  to  the  lum- 


328  COMPARATIVE   ZOOLOGY 

called  constant  temperatured  or  warm  blooded.  Other 
animals  do  not  vary  greatly  in  temperature  from  that 
of  their  surroundings,  and  are  called  changeable  tem- 
peratured or  cold  blooded.  Stilt,  their  temperature  does 
not  agree  exactly  with  that  of  the  air  or  water.  The 
bee  is  from  3°  to  10°,  and  the  earthworm  and  snail 
from  i  y  to  2°,  higher  than  the  air.  The  mean  temper- 
ature of  the  carp  and  toad  is  51°  ;  of  man,  98.5° ;  dog, 
99°;  cat,  101°;  squirrel,  105°;  swallow,  111°,  all  ac- 
cording to  the  Fahrenheit  scale. 


CHAPTER   XVIII* 
SECRETION  AND  EXCRETION 

IN  the  circulation  of  the  blood,  not  only  are  the 
nutrient  materials  taken  around  through  the  body  to  be 
used  in  the  construction  of  various  tissues,  but  certain 
special  fluids  are  taken  up  and  conveyed  to  the  external 
or  internal  surfaces  in  the  body,  where,  in  glandular 
structures,  further  elaboration  takes  place.  The  result- 
ing products  are  of  two  kinds :  some,  like  saliva,  gastric 
juice,  bile,  milk,  etc.,  are  for  useful  purposes ;  others, 
like  sweat  and  urine,  are  expelled  from  the  system  as 
useless  or  injurious.  The  separation  of  the  former  is 
called  secretion ;  the  removal  of  the  latter  is  excretion. 
Both  processes  are  substantially  alike. 

In  the  lower  forms,  there  are  no  special  organs,  but 
secretion  and  excretion  take  place  from  the  general 
surface.  The  simplest  form  of  a  secreting  organ  closely 
resembles  that  of  a  respiratory  organ,  a  thin  membrane 
separating  the  blood  from  the  cavity  into  which  the 
secretion  is  to  be  poured.  Usually,  however,  the  cells 
of  the  membrane  manufacture  the  secretion  from  ma- 
terials furnished  by  the  blood.  Even  in  the  higher 
animals,  there  are  such  secreting  membranes.  The 
membranes  lining  the  nose  and  alimentary  canal  and 
inclosing  the  lungs,  heart,  and  joints,  secrete  lubricating 
fluids. 

The  infolding  of  such  a  membrane  into  little  sacs  or 
short  tubes  (follicles),  each  having  its  own  outlet,  is  the 

*  See  Appendix. 
329 


330 


COMPARATIVE   ZOOLOGY 


type  of  all  secreting  and  excreting  organs.  The  lower 
animals  have  nothing  more  complex,  and  the  apparatus 
for  preparing  the  gastric  fluid  attains  no  further  devel- 
opment even  in  man.  When  a  cluster  of  these  follicles, 
or  sacs,  discharge  their  contents  by  one  common  duct, 

we  have  a  gland.  But 
whether  membrane,  folli- 
cle, or  gland,  the  organ 
is  covered  with  a  network 
of  blood  vessels,  and  lined 
with  epithelial  cells,  which 
are  the  real  agents  in  the 
process. 

The  Chief  Secreting  Or- 
gans are  the  salivary 
glands,  gastric  follicles, 
pancreas,  and  liver,  all 
situated  along  the  diges- 
tive tract. 

i.  The  salivary  glands, 

FIG.  286.  —  Three  plans  of  secreting  Mem-  which      Open      into      the 
.  branes.     The    heavy   line    represents    the 

areolar-vascular  layer;  the  next  line  is  the  mOUth,  SCCrCte  Saliva. 
basement,  or  limiting  membrane;  and  the  Thpv  pvici-  in  n<=»arl  T  all 
dotted  line  the  epithelial  layer:  a,  shows  ney  eXlbt  ]  nearly  ail 
increase  of  surface  by  simple  plaited  or  vertebrates,  hig her  mol- 
fringed  projections;  b,  five  modes  of  in- 
crease by  recesses,  forming  simple  glands,  lusks,  and  inSCCtS,  and  are 
or  follicles:  c,  two  forms  of  compound  .  ,  ,  .  ,  . 

glands.  most  largely  developed  in 

such  as  live  on  vegetable 

food.  The  saliva  serves  to  lubricate  or  dissolve  the 
food  for  swallowing,  and,  in  some  mammals,  aids  also 
in  digestion  of  starch.123 

2.  The  gastric  follicles  are  minute  tubes  in  the  walls 
of  the  stomach  secreting  gastric  juice.  They  are  found 
in  all  vertebrates,  and  in  the  higher  mollusks  and  arthro- 
pods. In  the  lower  forms,  a  simple  membrane  lined 
with  cells  serves  the  same  purpose.  Under  the  micro- 


SECRETION    AND    EXCRETION 


331 


scope,  the  soft  mucous  membrane  of  the  human  stomach 
presents  a  honeycomb  appearance,  caused  by  numerous 
depressions  or  cells.  At  the  bottom 
of  these  depressions  are  clusters  of 
spots,  which  are  the  orifices  of  the 
tubular  follicles.  The  follicles  are 
about  2^0  °f  an  mcn  m  diameter, 
and  number  millions. 

3.  The  pancreas,  or  "sweetbread," 
so  important   in  the  process  of   di- 
gestion, exists  in  all  but  the  lowest 
animals.      In  its  structure  it  closely 
resembles  the  salivary  glands.    In  the 
cuttlefish,  it  is  represented  by  a  sac ; 
in  fishes,  by  a  group  of  follicles.    It  is 
proportionally  largest  in  birds  whose 
salivary  glands  are  deficient.     The 
pancreatic  juice  enters  the  duodenum. 

4.  A  so-called  "  liver  "  is  found  in 
all  animals  having  a  distinct  diges- 
tive  cavity.      In   the   lower   animals    its   function  has 
been  shown  to  be  that  of  a  pancreas.     Thus,  in  polyps 

it  is  represented  by  yellowish  cells  lining  the 
stomach ;  in  insects,  by  cells  in  the  wall  of 
the  stomach  ;  in  mollusks,  by  a  cluster  of  sacs, 
or  follicles,  forming  a  loose  compound  gland. 
In  vertebrates,  a  true  liver,  the  larg- 
est gland  in  the  body,  is  well  defined, 
and  composed  of  a  mul- 
titude of  lobules  (which 
give   it   a    granular   ap- 
pearance)  arranged    on 
the  capillary  veins,  like 

FIG.  288. -Pancreas  of  Man;   o,  pancreas;  gt     grapCS     On     a     Stem,     and 
gall  bladder;  s.  cystic  duct;   c,  duct  from  the  .  i  J 

liver;  A  pyloric  valve;  e,f,  duodenum.  COn  tainin  g  H  UC  leat  ed 


FIG.  287.  —  Follicles  from  the 
Stomach  of  a  Dog,  X  150; 
near  the  mouth,  a,  there 
is  a  lining  of  columnar 
epithelium. 


332 


COMPARATIVE   ZOOLOGY 


secreting  cells.  It  is  of  variable  shape,  but  usually  two, 
three,  or  five  lobed,  and  is  centrally  situated  —  in  mam- 
mals, just  below  the  diaphragm.  In  most  vertebrates, 
there  is  an  appendage  to  the  liver  called  the  gall  bladder, 
which  is  simply  a  reservoir  for  the  bile. 


VP4 


FIG.  289.  —  Liver  of  the  Dog:  F,  F,  liver;  D,  duodenum  and  intestines;  P,  pancreas;  r> 
spleen;  e,  stomach,  f,  rectum;  R,  right  kidney;  B,  gall  bladder;  ch,  cystic  duct; 
F',  lobe  of  liver  dissected  to  show  distribution  of  portal  vein,  VP,  and  hepatic  vein, 
vh ;  */,  diaphragm;  VC,  venacava;  C,  heart. 

The  so-called  liver  of  invertebrates  is  more  like  the 
pancreas  of  vertebrates  in  function,  as  its  secretion 
digests  starches  and  albuminoids.  The  liver  of  verte- 
brates is  both  a  secretory  and  an  excretory  organ.  The 
bile  performs  an  important,  although  ill-understood,  func- 
tion in  digestion,  and  also  contains  some  waste  products. 


SECRETION   AND   EXCRETION  333 

The  gland  also  serves  to  form  sugar  (glycogen)  from 
part  of  the  digested  food,  and  may  well  be  called  a 
chemical  workshop  for  the  body.  In  animals  of  slow 
respiration,  as  crustaceans,  mollusks,  fishes,  and  reptiles, 
fat  accumulates  in  the  liver.  "Cod-liver  oil"  is  an 
example. 

The  Great  Excreting  Organs  are  the  lungs,  the  kidneys, 
and  the  skin  ;  and  the  substances  which  they  remove  from 
the  system  —  carbonic  acid,  water,  and  urea  —  are  the 
products  of  decomposition,  or  organic  matter  on  its  way 
back  to  the  mineral  kingdom.124  Different  as  these 
organs  appear,  they  are  constructed  upon  the  same 
principle :  each  consisting  of  a  very  thin  sheet  of  tissue 
separating  the  blood  to  be  purified  from  the  atmosphere, 
and  straining  out,  as  it  were,  the  noxious  matters.  All, 
moreover,  excrete  the  same  substances,  but  in  very  dif- 
ferent proportions  :  the  lungs  exhale  carbon  dioxide  and 
water,  with  a  trace  of  urea ;  the  kidneys  expel  water, 
urea',  and  a  little  carbon  dioxide ;  while  the  skin  par- 
takes of  the  nature  of  both,  for  it  is  not  only  respiratory, 
especially  among  the  lower  animals,  but  it  performs  part 
of  the  work  of  the  kidneys  in  case  they  are  diseased. 

1.  The  lungs  (and  likewise  gills)  are  mainly  excretory 
organs.     The  oxygen  they  impart  sweeps  with  the  blood 
through  every  part  of  the  body,  and  unites  with  the  tis- 
sues and  with  some  elements  of  the  blood.     Thus  are 
produced   heat  and  work,  whether  muscular,  nervous, 
secretory,  etc.     As   a  result  of   this  oxidation,  carbon 
dioxide,  water,  and   urea,  or   a   similar   substance,  are 
poured  into  the  blood.     The  carbon  dioxide  and  part  of 
the  water  are  passed  off  from  the  respiratory  organs. 
This  process  is  more  immediately  necessary  to  life  than 
any  other ;  the  arrest  of  respiration  is  fatal. 

2.  While  the  lungs  (and  skin  also,  to  a  slight  degree) 
are  sources  of  gain  as  well  as  loss  to  the  blood,  the  kid- 


334 


COMPARATIVE   ZOOLOGY 


neys  are  purely  excretory  organs.  Their  main  function 
is  to  eliminate  the  solid  products  of  decay  which  can  not 
pass  out  by  the  lungs.  In  mam- 
mals, they  are  discharged  in  solu- 
tion ;  but  from  other  animals 
which  drink  little  the  excretion 
is  more  or  less  solid.  In  insects, 
the  kidneys  are  groups  of  tubes 
(Figs.  239,  240);  in  the  higher 
mollusks,  they  are  represented 
by  spongy  masses  of  follicles 
(Fig.  244);  in  vertebrates,  they 
are  well-developed  glands,  two 
in -number,  and  consist  of  closely 
packed  tubes. 

3.  The  skin  of  the  soft-skinned 
animals,  particularly  of  amphibi- 
ans and  mammals,  is  covered  with 
minute  pores,  which  are  the  ends  of  as  many  delicate 
tubes  that  lie  coiled  up  into  a  knot  within  the  true  skin. 
These  are  the  sweat  glands,  which  excrete  water,  and 
with  it  certain  salts  and  gases. 

Besides  these  secretions  and  excretions,  there  are 
others,  confined  to  particular  animals,  and  designed  for 
special  purposes :  such  are  the  oily  matters  secreted 
from  the  skin  of  quadrupeds  for  lubricating  the  hair  and 
keeping  the  skin  flexible;  the  tears  of  reptiles,  birds, 
and  mammals;  the  milk  of  mammals';  the  ink  of  the 
cuttlefish  ;  the  poison  of  jelly  fishes,  insects,  and  snakes  ; 
and  the  silk  of  spiders  and  caterpillars. 


FIG.  290.  —  Section  of  Human 
Kidney,  showing  the  tubular 
portion,  3,  grouped  into  cones; 
7,  the  ureter,  or  outlet  for  the 
secretion. 


CHAPTER   XIX* 

THE  SKIN  AND  SKELETON 

The  Skin,  or  Integument,  is  that  layer  of  tissue  which 
covers  the  outer  surface  of  the  body.  The  term  Skele- 
ton is  applied  to  the  hard  parts  of  the  body,  whether 
external  or  internal,  which  serve  as  a  framework  or 
protection  to  the  softer  organs,  and  afford  points  of 
attachment  to  muscles.  If  external,  as  the  crust  of  the 
lobster,  it  is  called  exoskeleton  ;  if  internal,  as  the  bones 
of  man,  it  is  called  endoskeleton.  The  former  is  a 
modification  of  the  skin  ;  the  latter,  a  hardening  of  the 
deeper  tissues. 

i.  The  Skin.  —  In  the  lowest  forms  of  life,  as  amoeba, 
there  is  no  skin.  The  protoplasm  of  which  they  are 
composed  is  firmer  outside  than  inside,  but  no  mem- 
brane is  present.  In  Infusoria,  there  is  a  very  thin 
"cuticle"  covering  the  animal  (Fig.  9).  They  have 
thus  a  definite  form,  while  the  amcebas  continually 
change.  Sponges  and  hydras  also  have  no  true  skin. 
But  in  polyps,  the  outside  layer  of  the  animal  is  sepa- 
rated into  two  portions  —  ectoderm  and  endoderm 125  — 
which  may  be  regarded  as  partly  equivalent  to  epider- 
mis and  dermis  in  the  higher  animals.  These  two  layers 
are,  then,  generally  present.  The  outer  is  cellular,  the 
latter  fibrous,  and  may  contain  muscular  fibers,  blood 
vessels,  nerves,  touch  organs,  and  glands.  It  thus  be- 
comes very  complicated  in  some  animals. 

*  See  Appendix. 
335 


336  COMPARATIVE   ZOOLOGY 

In  worms  and  arthropods,  the  cellular  layer,  here 
called  hypodermis,  excretes  a  structureless  cuticle,  which 
may  become  very  thick,  as  in  the  tail  of  the  horseshoe 
crab,  or  may  be  hardened  by  deposition  of  lime  salts, 
as  in  many  Crustacea.  The  loose  skin,  called  the 
mantle,  which  envelops  the -body  of  the  mollusk,  corre- 
sponds to  the  true  skin  of  higher  animals.  The  border 
of  the  mantle  is  surrounded  with  a  delicate  fringe,  and, 
moreover,  contains  minute  glands,  which  secrete  the 
shell  and  the  coloring  matter  by  which  it  is  adorned. 
The  tunicates  have  a  leathery  epidermis,  remarkable 
for  containing  vegetable  cellulose  instead  of  lime. 

In  mammals,  whose  skin  is  most  fully  developed,  the 
dermis  is  a  sheet  of  tough  elastic  tissue,  consisting  of 
interlacing  fibers,  and  containing  blood,  vessels,  lym- 
phatics, sweat  glands,  and  nerves.  It  is  the  part  con- 
verted into  leather  when  hides  are  tanned,  and  attains 
the  extreme  thickness  of  three  inches  in  the  rhinoceros. 
The  upper  surface  in  parts  of  the  body  is  covered  with 
a  vast  number  of  minute  projections,  called  papilla, 
each  containing  the  termination  of  a, nerve;  these  are 
the  essential  agents  in  the  sense  of  touch126  (Fig.  345). 
They  are  best  seen  on  the  tongue  of  an  ox  or  cat,  arid 
on  the  human  fingers,  where  they  are  arranged  in  rows. 

Covering  this  sensitive  layer,  and  accurately  molded 
to  all  its  furrows  and  ridges,  lies  the  bloodless  and 
nerveless  epidermis.  It  is  that  part  of  the  skin  which 
is  raised  in  a  blister.  It  is  thickest  where  there  is  most 
pressure  or  hard  usage ;  on  the  back  of  the  camel  it 
attains  unusual  thickness.  The  lower  portion  of  the 
epidermis  (called  rete  mucosnm)  is  comparatively  soft, 
and  consists  of  nucleated  cells  containing  pigment  gran- 
ules, on  which  the  color  of  the  animal  depends.  Toward 
the  surface  the  cells  become  flattened,  and  finally,  on 
the  outside,  are  changed  to  horny  scales  (Fig.  199,  c). 


THE    SKIN    AND   SKELETON 


337 


These  scales  in  the  higher  animals  are  constantly 
wearing  off  in  the  form  of  scurf,  and  as  constantly  being 
renewed  from  below.  In  lizards  and  serpents,  the  old 
epidermis  is  cast  entire,  being  stripped  off  from  the 
head  to  the  tail ;  in  the  toad  it  comes  off  in  two  pieces ; 
in  the  frog,  in  shreds ;  in  fishes  and  some  mollusks,  in 
the  form  of  slime.  However  modified  the  epidermis, 
or  whatever  its  appendages,  the  like  process  of  removal 


FIG.  291.  — Section  of  Skin  from  Horse's  Nostril  (magnified) :  E,  epidermis;  D,  dermis; 
i,  horny  layer  of  epidermis;  2,  rete  mucosum;  3,  papillary  layer  of  dermis;  4,  ex- 
cretory duct  of  a  sudoriparous,  or  sweat,  gland;  5,  glomerule,  or  convoluted  tube  of 
the  same;  6,  hair  follicle;  7,  sebaceous  gland;  8,  internal  sheath  of  the  hair  follicle; 
9,  bulb  of  the  hair;  10,  mass  of  adipose  tissue. 

goes  on.  Mammals  shed  their  hair ;  birds,  their  feathers; 
and  crabs,  their  shells.  When  the  loss  is  periodical,  it 
is  termed  molting. 

2.  The  Skeletons.  —  (i)  The  Exoskeleton  is  developed  by 
the  hardening  of  the  skin,  and,  with  very  few  exceptions, 
is  the  only  kind  of  skeleton  possessed  by  invertebrate 
animals.  The  usual  forms  are  coral,  shells,  crusts, 
scales,  plates,  hairs,  and  feathers.  It  is  horny  or  cal- 
careous ;  while  the  endoskeleton  is  generally  a  deposit 
DODGE'S  GEN.  ZOOL.  —  22 


338 


COMPARATIVE  ZOOLOGY 


of  earthy  material  within  the  body,  and  is  nearly  con- 
fined to  the  vertebrates.  The  exoskeleton  may  be  of 
two  kinds — dermal  and  epidermal. 

Some  of  the  Protozoa,  as  Radiolaria  and  Foraminif- 
era,  possess  siliceous  and  calcareous  shells  of  the  most 
beautiful  patterns  (Fig.  2).  The  toilet  sponge  has  a 
skeleton  of  horny  fibers,  which  is  the  sponge  of  com- 
merce. Coral  is  the  solid  framework  of  certain  polyps. 
There  are  two  kinds :  one  represented  by  the  common 


FIG.  292. —  i,  Vertical  Section,  and,  2,  Transverse  Section,  of  a  sclerodermic  Corallite: 
a,  mouth;  b,  tentacles;  c,  stomach;  d,  intermesenteric  chamber;  e,  mesentery; 
_/",  septum;  g,  endoderm;  k,  epitheca;  k,  theca,  or  outer  wall;  in,  columella;  «, 
short  partitions;  /,  tabula,  or  transverse  partition;  r,  sclerobase;  s,  ccenenchyma, 
or  common  substance  connecting  neighboring  corallites;  /,  ectoderm;  x,  pali,  or  im- 
perfect partitions. 

white  coral,  which  is  a  calcareous  secretion  within  the 
body  of  the  polyp,  in  the  form  of  a  cylinder,  with  par- 
titions radiating  toward  a  center  (scleroderm)\  the 
other,  represented  by  the  solid  red  coral  of  jewelry,  is 
a  central  axis  deposited  by  a  group  of  polyps  on  the 
outside  (sclerobase}. 

The  skeleton  of  the  starfish  is  a  leathery  skin  in 
which  are  embedded  calcareous  particles  and  plates. 
The  sea  urchin  is  covered  with  an  inflexible  shell  of 
elaborate  and  beautiful  construction.  The  shell  is  really 
a  calcified  skin,  being  a  network  of  fibrous  tissue  and 


THE   SKIN   AND    SKELETON 


339 


FIG.  293.  —  Shell  of  Sea  Urchin  {Cidaris')  without  its  spines. 


earthy  plates.      It  varies  in   shape  from  a   sphere  to 

a   disk,   and   consists   of   hundreds   of   angular   pieces 

accurately  fitted 

together,   like 

mosaic    work. 

These  form  ten 

zones,    like    the 

ribs  of  a  melon, 

five   broad   ones 

alternating  with 

five    narrower 

ones.         The 

former   (called 

inter-  ambulacra) 

i        .   , 

are  covered  with 
tubercles  bearing  movable  spines.     The  narrow  zones 
(called  ambulacra,  as  they  are  likened  to  walks  through 

a  forest)  are 
pierced  with 
small  holes, 
through 
which  pro- 
ject fleshy 
sucker  feet. 

The  skin 
of  the  lobster 
is  hardened 
by  calcareous 
deposit  into 

FIG.  294.  —  Structure  of  Sea  Urchins'  Spines  (magnified)  :    i,  a,     a    "  CRISt,"    Or 
spine  of  Cidaris  cut  longitudinally;  /,  s,  ball-and-socket  joint;        ,      ..      *~.    , 
/,  pedicellariae  ;  2,  3,  transverse  sections  of  spines  of  Cidaris     Snell  J      '     DUt, 


instead     of 

forming  one  piece,  it  is  divided  into  a  series  of  seg- 
ments, which  move  on  each  other.  The  number  of  these 
segments,  or  rings,  is  usually  twenty  —  five  in  the  head, 


340 


COMPARATIVE   ZOOLOGY 


eight  in  the  thorax,  and  seven  in  the  abdomen.  In  the 
adult,  however,  the  rings  of  the  head  and  thorax  are 
often  soldered  together  into  one  shield,  called  cephalo- 
thorax ;  and  in  the  horseshoe  crab  the  abdominal  rings 
are  also  united.  The  shell  of  crustaceans  is  periodically 

cast  off,  for  the 
animals  continue 
to  grow  even  after 
they  have  reached 
their  mature  form. 
This  molting  is  a 
very  remarkable 
operation.  How 
the  lobster  can 
draw  its  legs 
from  their  cases 
without  unjoint- 
ing  or  splitting 
them  was  long  a 
puzzle.  The  flesh 
becomes  soft,  and 
is  drawn  through 

FIG.  295.  —  Diagram  of  an  Insect:    A,  head  bearing  the 

eyes  and  antennae;  B,  prothorax,  carrying  the  first  pair  the  joints,  th.6 
of  legs,  G;  C,  mesothorax,  carrying  the  second  pair  of  ,  . 

legs,  H,  and  first  pair  of  wings,  K;  D,  metathorax,  carry-  WOUndS  tilUS 
ing  the  third  pair  of  legs,  I,  and  second  pair  of  wings,  L;  caused  QUicklv 
E,  abdomen,  with  ovipositor,  F;  i,  coxa,  or  hip;  2,  tro-  J 

chanter;  3,  femur,  or  thigh;  4,  tibia,  or  shank;  5,  tarsus,      healing.  The 

or  foot;  6,  claw. 

cast-off     skeleton 

is  a  perfect  copy  of  the  animal,  retaining  in  their  places 
the  delicate  coverings  of  the  eyes  and  antennae,  and  even 
the  lining  membrane  of  the  stomach  with  its  teeth. 

The  horny  crust  of  insects  differs  from  that  of  crus- 
taceans in  consisting  mainly  of  a  horny  substance  called 
chitin  and  in  containing  no  lime.  The  head,  thorax,  and 
abdomen  are  distinct,  and  usually  consist  of  fourteen 
visible  segments  —  one  for  the  head,  three  for  the 


THE    SKIN    AND   SKELETON  341 

thorax  (called  prothorax,  mesothorax,  metathorax),  and 
ten  for  the  abdomen.  The  antennae,  or  feelers,  legs, 
and  wings,  as  well  as  hairs,  spines,  and  scales,  are 
appendages  of  the  skeleton.  As  insects  grow  only 
during  the  larval,  or  caterpillar,  state,  molting  is  con- 
fined to  that  period.  These  skeletons  are  epidermal, 
deposited  in  successive  layers,  from  the  inside,  and  are, 
therefore,  capable  of  but  slight  enlargement  when  once 
formed. 

The  shells  of  mollusks  are  well-known  examples  of 
exoskeletons.  The  mantle,  or  loose  skin,  of  these  ani- 
mals secretes  calcareous  earth  in  successive  layers,  con- 
verting the  epidermis  into  a  "  shell."  128  So  various  and 
characteristic  is  the  microscopic  character  of  shells, 
that  a  fragment  is  sometimes  sufficient  to  determine  the 
group  to  which  it  belongs.  Many  shells  resemble  that 
of  the  fresh-water  mussel  (  Unio),  which  is  composed  of 
three  parts :  the  external  brown  epidermis,  of  horny 
texture  ;  then  the  prismatic  portion,  consisting  of  minute 
columns  set  perpendicularly  to  the  surface;  and  the 
internal  nacreous  layer,  or  "  mother-of-pearl,"  made  up 
of  exceedingly  thin  plates.  The  pearly  luster  of  the 
last  is  due  to  light  falling  upon  the  outcropping  edges 
of  wavy  laminae.129  In  many  cases,'  the  prismatic  and 
nacreous  layers  are  traversed  by  minute  tubes.  Another 
typical  shell  structure  is  seen  in  the  common  cone,  a 
section  of  which  shows  three  layers,  besides  the  epi- 
dermis, consisting  of  minute  plates  set  at  different 
angles.  The  nautilus  shell  is  composed  of  two  distinct 
layers :  the  outer  one  having  the  fracture  of  broken 
china  ;  the  inner  one,  nacreous. 

Most  living  shells  are  made  of  one  piece,  as  the  snail; 
these  are  called  "  univalves."  Others,  as  the  clam,  con- 
sist of  two  parts,  and  are  called  "  bivalves."  In  either 
case,  a  valve  may  be  regarded  as  a  hollow  cone,  grow- 


342 


COMPARATIVE   ZOOLOGY 


ing  in  a  spiral  form.  The  ribs,  ridges,  or  spines  on  the 
outside  of  a  shell  mark  the  successive  periods  of  growth, 
and,  therefore,  correspond  to  the  age  of  the  animal. 
Figures  296  and  297  show  the  principal  parts  of  the 
ordinary  bivalves  and  univalves. 
The  valves  of  a  bivalve  are  gen- 
erally equal,  and  the  umbones,  or 
beaks,  a  little  in  front  of  the  cen-  x 
ter.  The  valves  are  bound  to- 


FlG.  296. —  Left  Valve  of  a  Bivalve  Mollusk  (Cy-  FIG.  297.  —  Section    of   a    Spiral 

therea   chione}:    h,   hinge   ligament;     «,    umbo;  Univalve  ( Triton  corrugatus} : 

/,   lunule;    c,   cardinal,    and   /,'  t' ',   lateral    teeth;  a,  apex;    b,    spire;    c,   suture; 

a,  a',  impressions   of  the   anterior   and   posterior  d,  posterior  canal;    e,  outer  lip 
adductor  muscles;  /,  pallial  impression;,  s,  sinus, 
occupied  by  the  retractor  of  the  siphons. 


of    the    aperture ;    f,    anterior 
canal. 


gether  by  a  ligature  near  the  umbones,  and  often,  also, 
by  means  of  a  "  hinge  "  formed  by  the  "  teeth  "  of  one 
valve  interlocking  into  cavities  in  the  other.  The  aper- 
ture of  a  univalve  is  frequently  closed  by  a  horny  or 
calcareous  plate,  called  "operculum,"  which  the  animal 
carries  on  the  back  of  the  hinder  portion  of  its  foot, 
and  which  is  a  part  of  the  exoskeleton.  The  shells  of 
mollusks  are  epidermal,  and  are,  therefore,  dead  and 
incapable  of  true  repair.  When  broken,  they  can  be 
mended  only  by  the  animal  pouring  out  lime  to  cement 


THE   SKIN   AND   SKELETON 


343 


the  parts  together.  They  can  not  grow  together,  like  a 
broken  bone. 

Embedded  in  the  back  of  the  cuttlefish  is  a  very  light 
spongy  "bone,"  which,  as  already  observed,  is  a  secre- 
tion from  the  skin,  and  therefere  belongs  to  the  exo- 
skeleton.  It  has  no  resemblance  to  true  bone,  but  is 
formed,  like  shells,  of  a  number  of  calcareous  plates. 
Nevertheless,  the  cuttlefish  does  exhibit  traces  of  an 
endoskeleton  :  these  are  plates  of  cartilage,  one  of  which 
surrounds  the  brain,  and  hence  may  be  called  a  skull. 
To  this  cartilage,  not  to  the  "cuttlebone,"  the  muscles 
are  attached. 

In  vertebrates,  the  exoskeleton  is  subordinate  to  the 
endoskeleton,  and  is  feebly  developed  in  comparison. 
It  is  represented  by 
a  great  variety  of  ap- 
pendages to  the  skin, 
which  are  mainly  or- 
gans for  protection, 
not  for  support. 
Some  are  horny 
outgrowths  of  the  epi- 
dermis, such  as  hairs, 
feathers,  nails,  claws, 
hoofs,  horns,  and  the 
scales  of  reptiles ; 

others  arise  from  the  hardening  of  the  dermis  by  cal- 
careous matter,  as  the  scales  of  fishes,  the  bony  plates 
of  crocodiles  and  turtles,  and  the  shield  of  the  armadillo. 

The  scales  of  fishes  (and  likewise  the  spines  of  their 
vertical  fins)  lie  embedded  in  the  overlapping  folds  of 
the  skin,  and  are  covered  with  a  thin,  slimy  epidermis. 
The  scales  of  the  bony  fishes  (perch,  salmon,  etc.)  con- 
sist of  two  layers,  slightly  calcareous,  and  marked  by 
concentric  and  radiating  lines.  Those  of  the  shark 


FIG.  298.  —  Skeletal  Architecture  in  the  Armadillo, 
showing  the  relation  of  the  carapax  to  the  verte- 
bral column. 


344 


COMPARATIVE   ZOOLOGY 


have  the  structure  of  teeth,  while  the  scutes,  or  plates,  of 
the  crocodiles,  turtles,  and  armadillos  are  of  true  bone. 

The  scales  of  snakes  and  lizards  are  horny  epidermal 
plates  covering  the  overlapping  folds  of  the  true  skin. 


FIG.  299.  —  Diagrammatic  Section  of  the  Skin  of  a  Fish  (Carp) :  a,  derm,  showing  lam- 
inated structure  with  vertical  fibers,  b;  c,  gristly  layer;  e,  laminated  layer,  with 
calcareous  granules;  d,  superficial  portion  developing  into  scales ;./,  scale  pit. 

In  some  turtles  these  plates  are  of  great  size,  and  are 
called  "  tortoise  shell " ;  they  cover  the  scutes.  The 
scales  on  the  legs  of  birds,  and  on  the  tail  of  the  beaver 
and  rat,  have  the  same  structure.  Nails  are  flattened 

horny  plates  de- 
veloped from  the 
upper  surface  of 
the  fingers  and 
toes.  Claws  are 
sharp  conical 
nails,  being  de- 
veloped from  the 
sides  as  well  as 
upper  surface; 
and  hoofs  are 
blunt  cylindrical 
claws.  Hollow 

FIG.  300. —Vertical  Section  of  the  Forefoot  of  the  Horse  homS,      aS     of     the 

(middle  digit):    i,  2,  4,  proximal,  middle,  and  distal,  ,        ,.,              _. 

or  ungual,  phalanges;   3,  sesamoid,  or  nut  bone;  5,  6,  7,  OX,  may  DC  llKCned 

tendons  ;    9   elastic  tissue  ;   8,  10,  internal  and  external  ^Q  daws  sheathing 
floor  ol  the  hoof;   n,  12,  internal  and  external  walls. 

a  bony  core.     The 

horn  of  the  rhinoceros  is  a  solid  mass  of  epidermal 
fibers.  "  Whalebone,"  the  rattle  of  the  rattlesnake,  and 
the  beaks  of  turtles  and  birds,  are  likewise  epidermal. 


THE   SKIN    AND    SKELETON 


345 


Hairs,  the  characteristic  clothing  of  mammals,  are 
elongated  horny  cones,  composed  of  "pith"  and  "crust." 
The  latter  is  an  outer  layer  of 
minute  overlapping  scales,  which 
are  directed  toward  the  point,  so 
that  rubbing  a  human  hair  or  fiber 
of  wool  between  the  thumb  and 
finger  pushes  the  root  end  away. 
The  root  is  bulbous,  and  is  con- 
tained in  a  minute  depression,  or 
sac,  formed  by  an  infolding  of  the 


FIG.  301.  —  Section  of  the  Root  and  part 
of  the  Shaft  of  a  Human  Hair,  highly 
magnified  :  it  is  covered  with  epidermic 
scales,  b,  the  inner  layer,  c,  forming  the 
outer  covering  of  the  shaft,  e,  being  im- 
bricated; the  root  consists  of  angular 
cells  loaded  with  pigment ;  d,  bulb. 


FIG.  302.  —  Parts  of  a  Feather: 
a,  quill,  or  barrel;  b,  shaft;  c, 
vane,  or  beard;  d,  accessory 
plume,  or  down ;  e,  f,  lower 
and  upper  umbilicus,  or  orifice, 
leading  to  the  interior  of  the 
quill. 


skin.  Hairs  are  usually  set  obliquely  into  the  skin. 
Porcupine's  quills  and  hedgehog's  spines  make  an  easy 
transition  to  feathers,  which  differ  from  hairs  only  in 
splitting  up  into  numerous  laminae.  They  are  the  most 


346  COMPARATIVE   ZOOLOGY 

complicated  of  all  the  modifications  of  the  epidermis. 
They  consist  of  a  "  quill "  (answering  to  the  bulb  of  a 
hair),  and  a  "shaft,"  supporting  the  "vane,"  which  is 
made  up  of  "barbs,"  "barbules,"  and  interlocking 
"processes."  The  quill  alone  is  hollow,  and  has  an  ori- 
fice at  each  end.  The  feather  is  molded  on  a  papilla, 
the  shaft  lying  in  a  groove  on  one  side  of  it,  and  the 
vane  wrapped  around  it.  When  the  feather  emerges 
from  the  skin,  it  unfolds  itself.  Thus  shaft  and  vanes 
together  resemble  the  quill  split  down  one  side  and 
spread  out. 

The  teeth  of  mollusks,  worms,  and  arthropods  are 
also  epidermal  structures.  Those  of  vertebrates  are 
mixed  in  their  origin,  the  dentine  being  derived  from 
the  dermis  and  the  enamel  from  the  epidermis.  In  all 
cases  teeth  belong  to  the  exoskeleton. 

(2)  The  Endoskeleton,  as  we  have  seen,  is  represented  in 
the  cuttlefish.  With  this  and  some  other  exceptions,  it 
is  peculiar  to  vertebrates.  In  the  cuttlefish,  and  some 
fishes,  as  the  sturgeon  and  shark,  it  consists  of  cartilage ; 
but  in  all  others  (when  adult)  it  is  bone  or  osseous 
tissue.  Yet  there  is  a  diversity  in  the  composition  of 
bony  skeletons ;  that  of  fresh-water  fishes  contains  the 
least  earthy  matter,  and  that  of  birds  the  most.  Hence 
the  density  and  ivory-whiteness  of  the  bones  of  the 
latter.  Unlike  the  shells  of  mollusks  and  the  crust  of 
the  lobster,  which  grow  by  the  addition  of  layers  to 
their  borders,  bones  are  moist,  living  parts,  penetrated 
by  blood  vessels  and  nerves,  and  covered  with  a  tough 
membrane,  called  periosteum,  for  the  attachrhent  of 
muscles. 

The  surface  of  bones  is  compact;  but  the  interior 
may  be  solid  or  spongy  (as  the  bones  of  fishes,  turtles, 
sloths,  and  whales),  or  hollow  (as  the  long  bones  of 
birds  and  the  active  quadrupeds).  There  are  also  cavi- 


THE   SKIN    AND   SKELETON  347 

ties  (called  sinuses)  between  the  inner  and  outer  walls 
of  the  skull,  as  is  remarkably  shown  by  the  elephant. 
The  cavities  in  the  long  bones  of  quadrupeds  are  filled 
with  marrow;  those  in  the  long  bones  of  most  birds 
and  in  skulls  contain  air. 

The  number  of  bones  not  only  differs  in  different 
animals,  but  varies  with  the  age  of  an  individual.  In 
very  early  life  there  are  no  bones  at  all ;  and  ossifica- 
tion, or  the  conversion  of  cartilage  into  bone,  is  not 
completed  until  maturity.  This  process  begins  at  a 
multitude  of  points,  and  theoretically  there  are  as 
many  bones  in  a  skeleton  as  centers  of  ossification. 
But  the  actual  number  is  usually  much  less  —  a  result 
of  the  tendency  of  these  centers  to  coalesce.  Thus,  the 
thigh  bone  in  youth  is  composed  of  five  distinct  por- 
tions, which  gradually  unite.  So  in  the  lower  verte- 
brates many  parts  remain  distinct  which  in  the  higher 
are  joined  into  one.  The  occiput  or  bone  at  the  base 
of  man's  skull  is  the  union  of  four  bones,  which  are 
seen  separate  in  the  skull  of  the  fish,  or  of  a  baby. 

A  complete  skeleton,  made  up  of  all  the  pieces  which 
might  enter  into  its  composition,  does  not  exist.  Every 
vertebrate  has  some  deficiency.  All,  except  amphioxus, 
have  a  skull  and  backbone ;  but  in  the  development  of 
the  various  parts,  and  especially  of  the  appendages, 
there  is  endless  variety.  Fishes  possess  a  great  number 
of  skull  bones,  but  have  no  fingers  and  toes.  The  snake 
has  plenty  of  ribs  and  tail,  but  no  breastbone ;  the  frog 
has  a  breastbone,  but  neither  tail  nor  ribs.  As  the  skele- 
ton of  a  fish  is  too  complicated  for  the  primary  student, 
we  will  select  for  illustration  the  skeleton  of  a  lion  — 
the  type  of  quadrupeds.  It  should  be  remembered, 
however,  that  all  vertebrates  are  formed  on  one  plan. 

In  the  lowest  vertebrate,  amphioxus,  the  only  skele- 
ton is  a  cartilaginous  rod  running  from  head  to  tail. 


348 


COMPARATIVE   ZOOLOGY 


There  is  no  skull,  nor  ribs,  nor  limbs.  In  the  carti- 
laginous fishes,  the  backbone  is  only  partially  ossified. 
But  usually  it  consists  of  a  number  of  separate  bones, 


called  vertebra,  arranged  along  the  axis  of  the  body. 
They  range  in  number  from  10  in  the  frog  to  305  in 
the  boa  constrictor.  The  skull,  with  its  appendages, 
and  the  vertebrae,  with  the  ribs  and  sternum,  make 


THE   SKIN   AND    SKELETON  349 

• 

up  the  axial  skeleton.  The  shoulder  and  pelvic  girdles 
and  the  skeleton  of  the  limbs  constitute  the  appendicular 
skeleton. 

A  typical  vertebra  consists  of  a  number  of  bony 
pieces  so  arranged  as  to  form  two  arches,  or  hoops, 
connected  by  a  central  bone,  or  centmm™  The  upper 
hoop  is  called  the  neural  arch,  because  it  encircles  the 
spinal  cord;  the  lower  hoop  is  called  the  hemal  arck, 
because  it  incloses  the  heart  and  the  great  central  blood 
vessels.  An  actual  vertebra,  however,  is  subject  to  so 


FIG.  304. — Vertebrae  —  A,  cervical;  B,  dorsal;  2,  centrum;  4,  transverse  process,  con- 
taining foramen,  a,  for  artery;  5,  articular  process;  3,  spinous  process,  or  neural 
spine;  i,  neural  canal;  6,  facets  for  head  of  rib,  the  tubercle  of  the  rib  fitting  in  a 
facet  on  the  process,  4;  b,  laminae,  or  neurapophyses. 

many  modifications,  that  it  deviates  more  or  less  from 
this  ideal  type.  Selecting  one  from  the  middle  of  the 
back  for  an  example,  we  see  that  the  centrum  sends^off 
from  its  dorsal  side  two  branches,  or  processes,  called 
neurapophyses.  These  meet  to  form  the  neural  arch, 
under  which  is  the  neural  canal,  and  above  which  is  a 
process  called  the  neural  spine.  On  the  anterior  and 
posterior  edges  of  the  arch  are  smooth  surfaces,  or 
zygapophyses,  which  in  the  natural  state  are  covered 
with  cartilage,  and  come  in  contact  with  the  correspond- 
ing surfaces  of  the  preceding  and  succeeding  vertebrae. 
The  bases  of  the  arch  are  notched  in  front  and  behind, 
so  that  when  two  vertebrae  are  put  together  a  round 


FIG.  307. 


THE   SKIN   AND   SKELETON 


351 


BONES  OF  THE  MAMMALIAN  SKULL* 


BRAIN   CASE 


NASAL. 

NOSE. 

ETHMOID. 


FRONTAL.  PARIETAL.  SUPRAOCCIPITAL. 

LAC  HRYMAL.  SQUAMOSAL. 

ORBITOSPHENOID.  EYE.  ALISPHEXOID.  PERI-  EAR.  OTIC.  EXOCCIPITAL. 

MALAR.  TYMPANIC.       . 

PRESPHENOID.  BASISPHENOID.  BASIOCCIPITAL. 


VOMER. 

PREMAXILLA.    MAXILLA.    PALATINE.    PTERYGO1D. 
LOWER  JAW,  OR  MANDIBLE. 


HYOID  ARCH. 


THE  SKULL  OF  THE  DOG 

FIG.  305. —Under  surface.  FIG.  306.  —  Upper  surface.  FIG.  307. — Longitudinal  ver- 
tical section;  one  half  natural  size.  S%)f  supraoccmital ;  .fi'.j^^exoccipital;  BO, 
basioccipital;  IP,  interparietal;  Pa,  parietal;  FV\  frontal;  Sff* squamosal ;  Ma, 
malar;  L,  lachrymal;  Afcrf/maxi\\a.;  PMx,  premaxilla;  Na,  nasal;  MT,  maxillo- 
turbinal;  ET,  ethmoturbinal;  ME,  ossified  portion  of  the  mesethmpid;  CE,  cri- 
briform, or  sievelike,  plate  of  the  ethmoturbinal;  l?£),  vomer;  &S,  presphenoid; 
OS,  orbitosphenoid  ;  ;4J>j"Sfisphenoid ;  Eg,  basisphenoid;  PI,  palatine;  PPj  ptery- 
goid;  Per,  periotic;  Ty,  tympanic  bulla;  an,  anterior  narial  aperture;  ap,  or  apf, 
anterior  palatine  foramen;  ppf,  posterior  palatine  foramen;  io,  infraorbital  foramen; 
J0/f'postorb\ta.\  process  of  frontal  bone  ;  ty,  optic  foramen;  sf,  sphenoidal  fissure; 
fr,  foramen  rotundum,  and  anterior  opening  of  alisphenoid  canal;  ^/"posterior 
opening  of  alisphenoid  canal;  fa,  foramen  ovale;  flm,  foramen  lacertim  medium; 
gf,  glenoid  fossa;  jtf',  postglenoid  process;  Plgf,  postglenoid  foramen;  earn,  external 
auditory  meatus;  sm,  stylomastoid  foramen;  flp,  foramen  lacerum  posterius;  cf, 
condylar  foramen;  pj/,  paroccipital  process;  otf,  occipital  condyle;  Jfrrt,  foramen 
magnum;  a,  angular  process;  s,  symphysis  of  the  mandible  where  it  unites  with  the 
left  ramus;  id,  inferior  dental  canal;  cd,  condyle;  cp,  coronoid  procass ;  z  indi- 
cates the  part  of  the  cranium  to  which  the  condyle  is  articulated  when  the  mandible 
is  in  place;  the  upper  border  in  which  the  teeth  are  implanted  is  called  alveolar;  sh, 
eh,  ch,  bh,  th,  hyoidean  apparatus,  or  os  lingua,  supporting  the  tongue.  In  the 
skulls  of  old  animals,  there  are  three  ridges:  occipital,  behind:  sagittal,  median,  on 
the  upper  surface;  and  superorbital,  across  the  frontal,  in  the  region  of  the  eye- 
brows. The  last  is  highly  developed  in  the  gorilla  and  other  apes. 


*  In  this  diagram,  modified  from  Huxley's,  the  italicized  bones  are  single;  the  rest 
are  double.  Those  in  the  line  of  the  Ethmoid  form  the  Cranio-facial  Axis;  these,  with 
the  other  sphenoids  and  occipitals,  are  developed  in  cartilage;  the  rest  are  membrane 
bones.  In  the  human  skull,  the  four  occipitals  coalesce  into  one 


352  COMPARATIVE   ZOOLOGY 

opening  (intervertebral  foramen}  appears  between  the 
pair,  giving  passage  to  the  nerves  issuing  from  the 
spinal  cord.  From  the  sides  of  the  arch,  blunt  trans- 
verse processes  project  outward  and  backward,  called 
diapophyses.  Such  are  the  main  elements  in  a  repre- 
sentative vertebra.  The  hemal  arch  is  not  formed  by 
any  part  of  the  vertebra,  but  by  the  ribs  and  breastbone. 
Theoretically,  however,  the  ribs  are  considered  as  elon- 
gated processes  from  the  centrum  (pleurapophyses),  and 
in  a  few  cases  a  hemal  spine  is  developed  corresponding 
to  the  neural  spine. 

The  vertebrae  are  united  together  by  ligaments,  but 
chiefly  by  a  very  tough,  dense,  and  elastic  substance  be- 
tween the  centra.  The  neural  arches  form  a  continuous 
canal  which  contains  and  protects  the  spinal  cord  ;  hence 
the  vertebral  column  is  called  the  neuroskeleton.  The 
column  is  always  more  or  less  curved ;  but  the  beautiful 
sigmoid  curvature  is  peculiar  to  man.  The  vertebrae 
gradually  increase  in  size  from  the  head  toward  the 
end  of  the  trunk,  and  then  diminish  to  the  end  of  the 
tail.  The  neural  arch  and  centrum  are  seldom  wanting ; 
the  first  vertebra  in  the  neck  has  no  centrum,  and  the 
last  in  the  tail  is  all  centrum.  The  vertebrae  of  the  ex- 
tremities (head  and  tail)  depart  most  widely  from  the 
typical  form. 

The  vertebral  column  in  fishes  and  snakes  is  divisible 
into  three  regions  —  head,  trunk,  and  tail.  In  the  higher 
animals  there  are  six  divisions  of  the  vertebral  column, 
the  skull,  and  cervical,  dorsal,  lumbar,  sacral,  and  caudal 
vertebra. 

The  skullul  is  formed  of  bones  whose  shape  varies 
greatly  from  that  of  typical  vertebrae.  The  number  of 
distinct  bones  composing  the  skull  is  greatest  in  fishes, 
and  least  in  birds ;  this  arises  partly  from  the  fact  that 
the  bones  remain  separate  in  the  former  case,  while 


THE   SKIN    AND    SKELETON 


353 


those  of  the  chick  become  united  together  (ancliylosed) 
in  the  full-grown  bird ;  but  many  bones  are  present  in 
the  fish  which  have  no  representatives  in  the  bird.  The 
skull  consists  of  the  brain  case  and  the  face.  The  prin- 
cipal parts  of  the  skull,  as  shown  in  the  dog's,  are : 
i.  The  occipital  bone  behind,  inclosing  a  large  hole,  or 
foramen  magnum,  on  each  side  of  which  are  rounded 
prominences,  called  condyles,  by  which  the  skull  articu- 


FIG.  308. —  Skull  of  the  Horse:  i,  premaxillary  bone;  2,  upper  incisors;  3,  upper 
canines;  4,  superior  maxillary;  5,  infraorbital  foramen;  6,  superior  maxillary  spine; 
7,  nasal  bones;  8,  lachrymal;  9,  orbital  cavity;  10,  lachrymal  fossa;  n,  malar; 
12,  upper  molars;  13,  frontal:  15,  zygomatic  arch;  16,  parietal;  17,  occipital  protu- 
berance; 18,  occipital  crest;  19,  occipital  condyles;  20,  styloid  processes;  21,  petrous 
bone;  22,  basilar  process;  23,  condyle  of  inferior  maxillary;  24,  parietal  crest;  25,  in- 
ferior maxillary  ;  26,  lower  molars;  27,  anterior  maxillary  foramen;  28,  lower  canines; 
29,  lower  incisors. 

lates  with  the  first  cervical  vertebra.  2.  The  two 
parietal  bones.  3.  The  two  frontal  bones.  These  five 
form  the  main  walls  of  the  skull.  4.  The  sphenoid,  on 
the  floor  of  the  skull  in  front  of  the  occipital,  and  Con- 
sisting of  six  pieces.  5.  The  two  temporal  bones,  in 
which  are  situated  the  ears.  In  man  each  temporal  is 
a  single  bone ;  but  in  most  animals  there  are  three  or 
more  —  the  periotic,  tympanic,  and  squamosal.  6.  The 
malars,  or  "  cheek  bones,"  each  of  which  sends' back  a 
•  DODGE'S  GEN.  zooi .  —  23 


354  COMPARATIVE   ZOOLOGY 

process  to  meet  one  from  the  squamosal,  forming  the 
zygomatic  arch.  7.  The  two  nasals,  forming  the  roof 
of  the  nose.  8.  The  two  maxillce,  that  part  of  the 
upper  jaw  in  which  the  canines,  premolars,  and  molars 
are  lodged.  9.  The  two  premaxillce,  in  which  the 
upper  incisors  are  situated.  10.  The  two  palatines, 
which,  with  the  maxillary  bones,  form  the  roof  of  the 
mouth.  There  are  two  appendages  to  the  skull :  the 
mandible,  or  lower  jaw,  whose  condyles,  or  rounded 
extremities,  fit  into  a  cavity  (the  glenoid)  in  the  tem- 
poral bone ;  and  the  hyoid  bone,  situated  at  the  root  of 
the  tongue. 

The  simplest  form  of  the  skull  is  a  cartilaginous  box, 
as  in  sharks,  inclosing  the  brain  and  supporting  the  car- 
tilaginous jaws  and  gill  arches.  In  higher  fishes  this 
box  is  overlaid  with  bony  plates  and  partly  ossified.  In 
frogs  the  skull  is  mainly  bony,  although  a  good  deal  of 
the  cartilage  remains  inside  the  bones.  In  higher  ver- 
tebrates the  cartilage  never  makes  an  entire  box,  and 
early  disappears. 

The  cervical  vertebra,  or  bones  of  the  neck,  are  pecul- 
iar in  having  an  orifice  on  each  side  of  the  centrum  for 
the  passage  of  an  artery.  The  first,  called  atlas,  because 
it  supports  the  head,  has  no  centrum,  and  turns  on  the 
second,  called  axis,  around  a  blunt  peglike  projection, 
called  the  odontoid  process.  The  centra  are  usually 
wider  than  deep,  and  the  neural  spines  very  short,  ex- 
cept on  the  last  one.  The  number  of  cervical  vertebrae 
ranges  from  I  in  the  frog  to  25  in  the  swan. 

The  dorsal  vertebra  are  such  as  bear  ribs,  which,  unit- 
ing with  the  breastbone,  or  sternum,  form  a  bony  arch 
over  the  heart  and  lungs,  called  the  thorax.  The  ster- 
num may  be  wanting,  as  in  fishes  and  snakes,  or  greatly 
developed,  as  in  birds.  When  present,  the  first  verte- 
bra whose  ribs  are  connected  with  it  is  the  first  dorsal. 


THE   SKIN   AND    SKELETON  355 

The  neural  spines  of  the  dorsal  series  are  generally  long, 
pointing  backward. 

The  lumbar  vertebra  are  the  massive  vertebrae  lying 
in  the  loins  between  the  dorsals  and  the  hip  bones. 

The  sacral  vertebra  lie  between  the  hip  bones,  and  are 
generally  consolidated  into  one  complex  bone,  called 
sacrum. 

The  caudal  vertebra  are  placed  behind  the  sacrum, 
and  form  the  tail.  They  diminish  in  size,  losing  pro- 
cesses and  neural  arch,  till  finally  nothing  is  left  but  the 
centrum.  They  number  from  3  or  4  in  man  to  270  in 
the  shark. 

Besides  the  lower  jaw,  hyoid,  and  ribs,  vertebrates 
have  other  appendages  to  the  spinal  column  —  two  pairs 
of  limbs ^  The  fore  limb  is  divided  into  the  pectoral 
arch  (or  shoulder  girdle),  the  arm,  and  the  hand.  The 
arch  is  fastened  to  the  ribs  and  vertebrae  by  powerful 
muscles,  and"  consists  of  three  bones,  the  scapula,  or 
shoulder  blade,  the  coracoid,  and  the  clavicle,  or  collar 
bone.  The  scapula  and  coracoid  are  generally  united 
in  mammals,  the  latter  being  a  process  of  the  former ; 
and  the  clavicles  are  frequently  wanting,  as  in  the  hoofed 
animals.  The  hjimerus,  radius,  and  ulna  are  the  bones 
of  the  arm,  the  first  articulating  by  ball  and  socket  joint 
with  the  scapula,  and  by  a  hinge  joint  with  the  radius 
and  ulna.  The  humerus  and  radius  are  always  present, 
but  the  ulna  may  be  absent.  The  bones  of  the  hand 
are  divided  into  those  of  the  carpus,  or  wrist ;  the  meta- 
carpus, or  palm ;  and  the  phalanges,  or  fingers.  The 
fingers,  or  " digits,"  range  in  number  from  i  to  5. 

The  hind  limb  is  composed  of  the  pelvic  arch  (or  hip 
bones),  the  leg,  and  the  foot.  These  parts  correspond 
closely  with  the  skeleton  of  the  fore  limb.  Like  the 
shoulder,  the  pelvic  arch,  or  os  innominatum,  consists  of 
three  bones  —  ilium,  ischium,  and  ptibis.  The  three  are 


356  COMPARATIVE   ZOOLOGY 

distinct  in  amphibians,  reptiles,  and  in  the  young  of 
higher  animals  ;  but  in  adult  birds  and  mammals  they 
become  united  together,  and  are  also  (except  in  whales) 
solidly  attached  to  the  sacrum.  The  two  pelvic  arches 
and  the  sacrum  thus  soldered  into  one  make  the  pelvis. 
The  leg  bones  consists  of  \hzfemur,  or  thigh  ;  the  tibia, 
or  shin  bone ;  and  the  fibula  or  splint  bone.  The 
rounded  head  of  the  femur  fits  into  a  cavity  (acetabulum) 
in  the  pelvic  arch,  while  the  lower  end  articulates  with 
the  tibia,  and  sometimes  (as  in  birds)  with  the  fibula 
also.  An  extra  bone,  the  patella,  or  kneepan,  is  hung 
in  a  tendon  in  front  of  the  joint  between  the  femur  and 
tibia  of  the  higher  animals.  The  foot  is  made  up  of  the 
tarsus,  or  ankle ;  the  metatarsus,  or  lower  instep ;  and 
faz  phalanges >  or  toes.  The  toes  number  from  i  in  the 
horse  to  5  in  man. 

Certain  parts  of  the  skeleton,  as  of  the  skull,  are 
firmly  joined  together  by  zigzag  edges  or  by  overlapping ; 
in  either  case  the  joint  is  called  a  suture.  But  the  great 
majority  of  the  bones  are  intended  to  move  one  upon 
another.  The  vertebrae  are  locked  together  by  their 
processes,  and  also  by  a  tough  fibrous  substance  between 
the  centra,  so  that  a  slight  motion  only  is  allowed.  The 
limbs  furnish  the  best  examples  of  movable  articulations, 
as  the  ball  and  socket  joint  at  the  shoulder,  and  the 
hinge  joint  at  the  elbow.  The  bones  are  held  together 
by  ligaments,  and  to  prevent  friction,  the  extremities  are 
covered  with  cartilage,  which  is  constantly  lubricated 
with  an  unctuous  fluid  called  synovia. 

A  chemical  analysis  of  bone  shows  it  to  consist  mainly 
of  phosphate  and  carbonate  of  lime  and  phosphate  of 
magnesia  mingled  with  glutin,  chondrin,  and  oil,  the 
amount  of  each  varying  in  different  animals. 


358 


COMPARATIVE   ZOOLOGY 


THE   SKIN   AND    SKELETON 


359 


FIG.  312.  —  Skeleton  of  the  Tortoise  (plastron  removed) :  a,  cervical  vertebrae;  c,  dorsal 
vertebrae;  d,  ribs;  e,  marginal  bones  of  the  carapace;  /,  scapula;  k,  precoracoid; 
b,  coracoid;  f,  pelvis;  /,  femur;  gt  tibia;  h,  fibula. 


FIG.  313.  —  Skeleton  of  a  Vulture:  i,  cranium  —  the  parts  of  which  are  separable  only  in 
the  chick;  2,  cervical  vertebrae;  3,  dorsal;  4,  coccygeal,  or  caudal;  the  lumbar  and 
sacral  are  consolidated ;  5,  ribs ;  6,  sternum,  or  breastbone,  extraordinarily  developed ; 
7,  furculum,  clavicle,  or  "wishbone";  8,  coracoid;  9,  scapula;  10,  humerus;  u, 
ulna,  with  rudimentary  radius;  12,  metacarpals;  13,  phalanges  of  the  great  digit  of 
the  wing;  19,  thumb;  14,  pelvis;  15,  femur;  16,  tibiatarsus  and  fibula,  or  cms ;  17, 
tarsometatarsus ;  18,  internal  digit,  or  toe,  formed  of  three  phalanges;  the  middle  toe 
has  four  phalanges ;  the  outer,  five ;  and  the  back  toe,  or  thumb,  two. 


360 


COMPARATIVE  ZOOLOGY 


FIG.  314.  —  Skeleton  of  the  Horse  (Equus  caballus):  22,  premaxillary;  12,  foramen  in 
the  maxillary;  15,  nasal;  9,  orbit;  19,  coronoid  process  of  lower  jaw;  17,  surface  of 
implantation  for  the  masseter  muscle ;  there  are  seven  cervical  vertebrae,  nineteen 
dorsal,  D  —  D;  five  lumbar,  a-e ;  five  sacral,/^//  and  seventeen  caudal, />-?•/  51, 
scapula,  or  shoulder  blade  ;  /,  spine,  or  crest;  //;  coracoid  process  (acromion  wanting) ; 
i  >  first"  pair  of  ribs  (clavicle  wanting,  as  in  all  Ungulates);  e,  sternum;  a,  shaft  of 
humerus;  b,  deltoid  ridge;  g  head  fitting  in  the  glenoid  cavity  of  the  scapula  —  near 
it  is  a  great  tuberosity  for  the  attachment  of  a  powerful  muscle;  k,  condyles;  54, 
radius,  to  which  is  firmly  anchylosed  a  rudimentary  ulna,  55,  the  olecranon;  56,  the 
seven  bones  of  the  carpus,  or  wrist;  57,  large  metacarpal,  or  "  cannon  bone,"  with 
two  "splint  bones";  58,  fetlock  joint;  59,  phalanges  of  the  developed  digit,  corre- 
sponding to  the  third  finger  in  man;  62,  pelvis;  63,  the  great  trochanter,  or  prom- 
inence on  the  femur,  65;  66,  tibia;  67,  rudimentary  fibula;  68,  hock,  or  heel,  falsely 
called  knee;  69,  metatarsals. 


THE   SKIN  AND   SKELETON  361 


FIG.  315.  —  Skeleton  of  the  Cow  (Bos  ta-urus). 


FIG.  316.  —  Skeleton  of  an  Elephant  {Elephas  indicus). 


362 


COMPARATIVE   ZOOLOGY 


FIG.  317.  —  Skeleton  of  the  Chimpanzee  (Anthropopithecus  troglodytes). 


CHAPTER   XX* 

HOW   ANIMALS   MOVE 

i.  The  power  of  animal  motion  is  vested  in  proto- 
plasm, cilia,  and  muscles.  The  power  of  contractility 
is  one  of  the  fundamental  physiological  properties  of 
protoplasm,  like  sensibility  and  the  power  of  assimila- 
tion. Protoplasmic  animals,  like  the  amoeba  and  Rhiz- 
opoda  (Figs.  I,  213),  move  by  the  contractility  of  their 
protoplasm,  as  also  may  the  embryos  of  higher  animals 
upon  the  yolk  of  the  egg.  Protoplasm  may  be  extended 
into  projections  called  pseudopodia,  by  whose  contrac- 
tion the  animal  may  move. 

Infusoria,  and  nearly  all  higher  animals,  possess  cilia 
(Figs.  9,  1 1 ).  These  are  short  microscopic  threads  of 
protoplasm  which  have  the  power  of  bending  into  a 
sickle  shape  and  straightening  out.  As  they  bend  much 
faster  than  they  straighten,  and  as  they  all  work  together, 
they  can  cause  motion  of  the  animal,  or  may  serve  to 
produce  currents  in  the  water,  the  animal  remaining  at 
rest.  They  are  seen  on  the  outside  of  Infusoria,  and 
of  embryos  of  very  many  higher  animals,  serving  as 
paddles  for  locomotion ;  they  line  the  channels  in  the 
gills  of  the  oyster,  creating  currents  for  respiration ; 
and  they  cover  the  walls  of  the  passages  to  our  lungs  to 
expel  the  mucus.  Flagella  (Figs.  4,  5,  6)  are  a  sort  of 
long  cilia,  which  are  thrown  into  several  curves  when 
active,  resembling  a  whiplash,  whence  their  name.  Both 
cilia  and  flagella  seem  to  be  wanting  in  arthropods. 

*  See  Appendix. 
363 


364 


COMPARATIVE   ZOOLOGY 


:Dytiscus. 


The  cause  of  ciliary  motion  is  unknown.  One-sided 
contraction  is  their  property,  as  straight  contraction 
is  distinctive  of  the  muscle  fiber.  No  structure  can, 
however,  be  seen  in  them  with  the  micro- 
scope. No  nerves  go  to  them,  yet  they 
work  in  concert,  waves  of  motion  passing 
over  a  surface  covered  with  cilia,  as  over 
a  field  of  grain  moved  by  the  wind. 

i.  Muscle.  —  Muscular  tissue  is  the  great 
FIG  i  -waves  motor  agent,  and  exists  in  all  animals  from 
of  Contraction  the  coral  to  man.133  The  power  of  con- 
tractility, which  in  the  amoeba  is  diffused 
throughout  the  body,  is  here  confined  to 
bundles  of  highly  elastic  fibers,  called  muscles.  When  a 
muscle  contracts,  it  tends  to  bring  its  two  ends  together, 
thus  shortening  itself,  at  the  same  time  increasing  in 
thickness.  This  shrinking  property  is  excited  by  exter- 
nal stimulants,  such  as  electricity,  acids,  alkalies,  sudden 
heat  or  cold,  and  even  a  sharp  blow ;  but 
the  ordinary  cause  of  contraction  is  an 
influence  from  the  brain  conveyed  by  a 
nerve.  The  property,  however,  is  inde- 
pendent of  the  nervous  system,  for  the 
muscle  may  be  directly  stimulated.  The 
amount  of  force  with  which  a  muscle 
contracts  depends  on  the  number  of  its 
fibers;  and  the  amount  of  shortening,  on 
their  length. 

As  a   rule,   muscles   are  white  in  cold- 
blooded   animals,    and   red    in    the   warm-   FIG. 
blooded.       They    are    white     in     all     the 
invertebrates,  fishes,  batrachians,  and  rep- 
tiles, except  salmon,  sturgeon,  and  shark ; 
and  red  in  birds  and   mammals,  except  in  the  breast 
of  the  common  fowl,  and  the  like.134 


319.  — Un- 
striped  Muscu- 
lar Fiber,  much 
enlarged;  «, nu- 
cleus. 


HOW  ANIMALS    MOVE  365 

It  is  also  a  rule,  with  some  exceptions,  that  the  volun- 
tary muscles  of  vertebrates,  and  all  the  muscles  of  the 
lobster,  spider,  and  insect  tribes,  are  striated ;  while  the 
involuntary  muscles  of  vertebrates,  and  all  the  muscles 
of  radiates,  worms,  and  mollusks,  are  smooth.  All  mus- 
cles attached  to  internal  bones,  or  to  a  jointed  external 
skeleton,  are  striated.  The  voluntary  muscles  of  verte- 
brates are  generally  solid,  and  the  involuntary  surround 
cavities.135 

This  leads  to  another  classification  of  muscles :  into 
those  which  are  attached  to  solid  parts  within  the  body ; 
those  which  are  attached  to  the  skin  or  its  modifications; 
and  those  having  no  attachments,  being  complete  in 
themselves.  The  last  are  hollow  or  circular  muscles, 
inclosing  a  cavity  or  space,  which  they  reduce  by  con- 
traction. Examples  of  such  are  seen  in  the  heart,  blood 
vessels,  stomach,  iris  of  the  eye,  and  around  the  mouth. 
In  the  lower  invertebrates,  the  muscular  system  is  a  net- 
work of  longitudinal,  transverse,  and  oblique  fibers  inti- 
mately blended  with  the  skin,  and  not  divisible  into  sep- 
arate muscles.  As  in  the  walls  of  the  human  stomach, 
the  fibers  are  usually  in  distinct  layers.  This  ar- 
rangement is  exhibited  by  soft-bodied  animals,  like 
the  sea  anemone,  the  snail,  and  the  earthworm.  Four 
thousand  muscles  have  been  counted  in  a  caterpillar. 
There  are  also  "  skin  muscles  "  in  the  higher  animals, 
as  those  by  which  the  horse  produces  a  twitching  of  the 
skin  to  shake  off  insects,  and  those  by  which  the  hairs 
of  the  head  and  the  feathers  of  birds  are  made  to  stand 
on  end.  Invertebrates  whose  skin  is  hardened  into  a 
shell  or  crust  have  muscles  attached  to  the  inside  of  such 
a  skeleton.  Thus,  the  oyster  has  a  mass  of  parallel 
fibers  connecting  its  two  valves ;  while  in  the  lobster 
and  bee  fibers  go  from  ring  to  ring,  both  longitudinally 
and  spirally.  The  muscles  of  all  invertebrates  are 


366  COMPARATIVE   ZOOLOGY 

straight  parallel  fibers,  not  in  bundles,  but  distinct,  and 
usually  flat,  thin,  and  soft. 

The  great  majority  of  the  muscles  of  vertebrates  are 
attached  to  the  bones,  and  such  are  voluntary.  The 
fibers,  which  are  coarsest  in  fishes  (most  of  all  in  the 
rays),  and  finest  in  birds,  are  bound  into  bundles  by 
connective  tissue ;  and  the  muscles  thus  made  up  are 
arranged  in  layers  around  the  skeleton.  Sometimes 
their  extremities  are  attached  to  the  bones  (or  rather  to 
the  periosteum)  directly ;  but  generally  by  means  of 
white  inelastic  cords,  called  tendons.  In  fishes,  the 
chief  masses  of  muscle  are  disposed  along  the  sides 
of  the  body,  apparently  in  longitudinal  bands,  reaching 
from  head  to  tail,  but  really  in  a  series  of  vertical  flakes, 
one  for  each  vertebra.  In  proportion  as  limbs  are  de- 
veloped, we  find  the  muscles  concentrated  about  the 
shoulders  and  hips,  as  in  quadrupeds.  The  bones  of 
the  limbs  are  used  as  levers  in  locomotion,  the  fulcrum 
being  the  end  of  a  bone  with  which  the  moving  one  is 
articulated.  Thus,  in  raising  the  arm,  the  humerus  is  a 
lever  working  upon  the  scapula  as  a  fulcrum.  The 
most  important  muscles  are  called  extensors  and  flexors. 
The  latter  are  such  as  bring  a  bone  into  an  angle  with 
its  fulcrum  —  as  in  bending  the  arm  —  while  the  former 
straighten  the  limb.  Abductors  draw  a  limb  away  from 
the  middle  line  of  the  body,  or  a  finger  or  toe  away 
from  the  axis  of  the  limb,  while  adductors  bring  them 
back. 

2.  Locomotion.  —  All  animals  have  the  power  of  vol- 
untary motion,  and  all,  at  one  time  or  another,  have  the 
means  of  moving  themselves  from  place  to  place.  Some 
are  free  in  the  embryo  life,  and  fixed  when  adult,  as 
the  sponge,  coral,  crinoid,  and  oyster.  There  may  be 
no  regular,  well-defined  means  of  progression,  as  in  the 
amoeba,  which  extemporizes  arms  to  creep  over  the  sur- 


HOW  ANIMALS   MOVE  367 

face ;  or  movement  may  be  accomplished  by  the  con- 
traction of  the  whole  body,  as  in  the  jellyfish,  which, 
pulsating  about  fifteen  times  in  a  minute,  propels  itself 
through  the  water.  So  the  worms  and  snakes  swim  by 
the  undulations  of  the  body. 

But  as  a  rule,  animals  are  provided  with  special 
organs  for  locomotion.  These  become  reduced  in  num- 
ber, and  progressively  perfected,  as  we  advance  in  the 
scale  of  rank.  Thus,  the  infusorian  is  covered  with 
thousands  of  hairlike  cilia;  the  starfish  has  hundreds 
of  soft,  unjointed,  tubular  suckers ;  the  centipede  has 
from  30  to  40  jointed  hollow  legs;  the  lobster,  10;  the 
spider,  8  ;  and  the  insect,  6 ;  the  quadruped  has  4  solid 
limbs  for  locomotion  ;  and  man,  only  2. 

(i)  Locomotion  in  Water.  —  As  only  the  lower  forms  of 
life  are  aquatic,  and  as  the  weight  of  the  body  is  partly 
sustained  by  the  element,  we  must  expect  to  find  the  or- 


FIG.  320.  —The  Fins  of  a  Fish  (Pike  Perch}. 

gans  of  progression  simple  and  feeble.  The  Infusoria 
swim  with  great  rapidity  by  the  incessant  vibrations  of 
the  delicate  filaments,  or  cilia,  on  their  bodies.  The 
common  squid  on  our  coast  admits  water  into  the  inte- 
rior of  the  body,  and  then  suddenly  forces  it  out  through 
a  funnel,  and  thus  moves  backward,  or  forward,  or 
around,  according  as  the  funnel  is  turned  —  toward  the 
head,  or  tail,  or  to  one  side.  The  lobster  has  a  fin  at 


368 


COMPARATIVE   ZOOLOGY 


the  end  of  its  tail,  and  propels  itself  backward  by 
a  quick  downward  and  forward  stroke  of  the  ab- 
domen. 

But  fishes,  whose  bodies  offer  the  least  resistance  to 
progression  through  water,  are  the  most  perfect  swim- 
mers. Thus,  the  salmon  can  go  twenty  miles  an  hour, 
and  even  ascend  cataracts.  They  have  fins  of  two 
kinds :  those  set  obliquely  to  the  body,  and  in  pairs ; 
and  those  which  are  vertical,  and  single.  The  former, 
called  pectoral  and  ventral  fins,  rep- 
resent the  fore  and  hind  limbs  of 
quadrupeds.  The  vertical  fins,  which 
are  only  expansions  of  the  skin,  vary 
in  number ;  but  in  most  fishes  there 
are  at  least  three :  the  caudal,  or 
tail  fin  ;  the  dorsal,  or  back  fin  ;  and 
the  anal,  situated  on  the  abdomen, 
near  the  tail  The  chief  locomotive 
agent  is  the  tail,  which  sculls  like  a 
stern  oar ;  the  other  fins  are  mainly 
used  to  balance  and  raise  the  body. 
When  the  two  lobes  of  the  tail 
are  equal,  and  the  vertebral  column 
stops  near  its  base,  as  in  the  trout, 
it  is  said  to  be  homo  cereal.  If  the 
vertebrae  extend  into  the  upper 
lobe,  making  it  longer  than  the  lower  one,  as  in  the 
shark,  the  tail  is  called  heterocercal.  The  latter  is  the 
more  effective  for  varying  the  course ;  the  shark,  e.g., 
will  accompany  and  gambol  around  a  ship  in  full  sail 
across  the  Atlantic.  The  whale  swims  by  striking  the 
water  up  and  down,  instead  of  laterally,  with  a  finlike 
horizontal  tail.  Many  air-breathing  animals  swim  with 
facility  on  the  surface,  as  the  water  birds,  having  webbed 
toes,  and  most  of  the  reptiles  and  quadrupeds. 


FIG.  321.  —  Diagram  illustrat- 
ing the  locomotion  of  a 
Fish.  The  tail  describes 
the  arc  of  an  ellipse;  the 
resultant  of  the  two  im- 
pulses is  the  straight  line 
in  front. 


HOW  ANIMALS   MOVE 


369 


(2)  Locomotion  in  Air.  —  The  power  of  flight  requires  a 
special  modification  of  structure  and  an  extraordinary 
muscular  effort,  for  air  is  800  times  lighter  than  water. 
Nevertheless,  the  velocity  attainable  by  certain  birds  is 
greater  than  that  of  any  fish  or  quadruped ;  the  hawk 
being  able  to  go  at  the  rate  of  1 50  miles  an  hour.  The 
bodies  of  insects  and  birds  are  made  as  light  as  possible 
by  the  distribution  of  air  sacs  or  air  cavities.136 

The  wings  of  insects  are  generally  four  in  number ; 
sometimes  only  two,  as  in  the  fly.  .  They  are  moved  by 
muscles  lying  inside  the  thorax.  They  are  simple  ex- 
pansions of  the  skin,  or  crust,  being  composed  of  two 
delicate  films  of  the  epidermis  stretched  upon  a  network 
of  tubes.  There  are  three  main  varieties :  thin  and 
transparent,  as  in  the  dragon  fly ;  opaque,  and  covered 
with  minute  colored  scales,  which  are  in  reality  flattened 
hairs,  as  in  the  butterfly ;  and  hard  and  opaque,  as  the 
first  pair  (called  elytra)  of  the  beetle. 

The  wings  of  birds,  on  the  other  hand,  are  modified 
fore  limbs,  consisting  of  three  sets  of  feathers  (called 


FIG.  322.  —  Flamingoes  taking  Wing. 

primary,  secondary,  and  tertiary),  inserted  on  the  hand, 
forearm,  and  arm.     The  muscles  which  give  the  down- 
ward stroke  of  the  wing  are  fastened  to  the  breastbone ; 
DODGE'S  GEN.  ZOO'L.  —  24 


370 


COMPARATIVE   ZOOLOGY 


arid  their  power,  in  proportion  to  the  weight  of  the  bird, 
is  very  great.  Yet  the  insect  is  even  superior  in  vigor 
and  velocity  of  flight.137  In  ascending,  the  bird  slightly 
rotates  the  wing,  striking  downward  and  a  little  back- 
ward ;  while  the  tail  acts  as  a  rudder.  A  short,  rounded, 
concave  wing,  as  in  the  common  fowl,  is  not  so  well 
fitted  for  high  and  prolonged  flight  as  the  long,  broad, 
pointed,  and  flat  wing  of  the  eagle.  The  wing  is  folded 
by  means  of  an  elastic  skin  and  muscle  connecting  the 
shoulder  and  wrist.  Besides  insects  and  birds,  a  few 
other  animals  have  the  power  of  flight,  as  bats,  by 
means  of  long  webbed  fingers ;  flying  fishes,  by  large 
pectoral  fins.  Flying  reptiles,  flying  squirrels,  and  the 
like,  have  a  membrane  stretched  on  the  long  ribs,  or 
connecting  the  fore  and  hind  limbs,  which  they  use  as  a 
parachute,  enabling  them  to  take  very  long  leaps. 

(3)  Locomotion  on  Solids.  —  This  requires  less  muscular 
effort  than  swimming  or  flying.  The  more  unyielding 
the  basis  of  support,  the  greater  the  amount  of  power 
left  to  move  the  animal  along.  The  simplest  method  is 


FIG.  323.  —  Diagrammatic  Section  of  the  Disk  and  one  Ray  of  Starfish:  a,  mouth;  b, 
stomach;  c,  hepatic  caecum;  d,  dorsal  or  aboral  surface;  e,  ambulacral  plates;  f> 
ovary;  gt  tubular  feet;  k,  internal  sacs  for  distending  the  feet. 

the  suctorial,  the  animal  attaching  itself  to  some  fixed 
object,  and  then,  by  contraction,  dragging  the  body  on- 
ward. But  the  higher  and  more  common  method  is  by 
the  use  of  bones,  or  other  hard  parts,  as  levers. 


HOW  ANIMALS    MOVE  3/1 

The  starfish  creeps  by  the  working  of  hundreds  of 
tubular  suckers,  which  are  extended  by  being  filled  with 
fluid  forced  into  them  by  little  sacs.  The  clam  moves 
by  fixing  and  contracting  a  muscular  appendage,  called 
a  "foot."  The  snail  has  innumerable  short  muscles  on 
the  under  side  of  its  body,  which,  by  successive  contrac- 
tions, resembling  minute  undulations,  enable  the  animal 
to  glide  forward  apparently  without  effort.  The  leech 
has  a  sucker  at  each  end ;  fixing  itself  by  the  one  on 
its  tail,  and  then  stretching  the  body,  by  contracting  the 
muscular  fibers  which  run  around  it,  the  creature  fastens 
its  mouth  by  suction,  and  draws  forward  the  hinder 
parts  by  the  contraction  of  longitudinal  muscles.  The 
earthworm  lengthens  and  shortens  itself  in  the  same 
way  as  the  leech,  but  instead  of  suckers  for  holding  its 
position,  it  has  numerous  minute  spines  which  may  be 
pointed  backward  or  forward ;  while  the  caterpillar  has 
short  legs  for  the  same  purpose.  The  legless  serpent 
moves  by  means  of  the  scutes,  or  large  scales  on  the 
under  side  of  the  body,  acted  upon  by  the  ribs.  In  a 
straight  line,  locomotion  is  slow ;  but  by  curving  the 
body,  laterally  or  vertically,  it  can  glide  or  Jeap  with 
great  rapidity. 

Most  animals  have  movable  jointed  limbs,  acted  upon 
as  levers  by  numerous  muscles.  The  centipede  has 
forty-two  legs,  each  with  five  joints  and  a  claw.  The 
crab  has  five  pairs  of  six-jointed  legs  ;  but  the  front 
pair  is  modified  into  pincers  for  prehension.  With  the 
rest,  which  end  in  a  sharp  claw,  the  crab  moves  back- 
ward, forward,  or  sideways.  The  spider  has  eight  legs, 
usually  seven-jointed,  and  terminating  in  two  claws 
toothed  like  a  comb,  and  a  third  which  acts  like  a 
thumb.  In  running,  it  moves  the  first  right  leg,  then 
the  fourth  left ;  next,  the  first  left,  and  then  the  fourth 
right;  then  the  third  right  and  second  left  together; 


372 


COMPARATIVE   ZOOLOGY 


and  lastly,  the  third  left  and  second  right  together. 
The  front  and  hind  pairs  are,  therefore,  moved  like 
those  of  a  quadruped.  The  insect  has  six  legs,  each 
of  five  parts:  the  coxa ;  trochanter ;  femur;  tibia,  or 

shank ;  and  tarsus. 
The  last  is  sub- 
divided usually  into 
five  joints  and  a 
pair  of  claws. 
Such  as  can  walk 
upside  down,  as 
the  fly,  have,  in 
addition,  two  or 
three  pads  between 
the  claws.138  These 
pads  bear  hairs 
which  secrete  a 
sticky  fluid  by 
means  of  which 
the  fly  adheres  to 
the  surface.  While 
the  leg  bones  of 
vertebrates  are 
covered  by  the  muscles  which  move  them,  the  limbs 
of  insects  are  hollow,  and  the  muscles  inside.  The  fore 
legs  are  directed  forward,  and  the  two  hinder  pairs 
backward.  In  motion,  the  fore  and  hind  feet  on  one 
side,  and  the  middle  one  on  the  other,  are  moved  simul- 
taneously, and  then  the  remaining  three. 

The  four-legged  animals  have  essentially  the  same 
apparatus  and  method  of  motion.  The  crocodile  has 
an  awkward  gait,  owing  to  the  fact  that  the  limbs  are 
short,  and  placed  far  apart,  so  that  the  muscles  act  at 
a  mechanical  disadvantage.  The  tortoise  is  proverbially 
slow,  for  a  similar  reason.  Both  swim  better  than  they 


FIG.  324.  —  Feet  of  Insects,  magnified:  A,  Bibiofebrilis; 
B,  House  Fly  (Musca  domestic  a) ;  C,  Water  Beetle 
(Dytiscus). 


HOW  ANIMALS   MOVE 


373 


walk.     Lizards  are  light  and  agile,  but  progression  is 
aided  by  a  wriggling  of  the  body. 

The  locomotive  organs  of  the  mammalian  quadrupeds 
are  much  more  highly  organized.  The  bones  are  more 
compact ;  the  vertebral  column  is  arched  and  yet  elastic, 
between  the  shoulder  and  hip,  and  the  limbs  are  placed 
vertically  underneath  the  body.  The  bones  of  the  fore 
limb  are  nearly  in  a  line ;  but  those  of  the  hind  limb, 
which  is  mainly  used  to  project  the  body  forward,  are 
more  or  less  inclined  to  one  another,  the  angle  being 


^ji^ 

FIG.  325.  —  Feet  of  Carnivores :  A,  Plantigrade  (Bear);  B,  Pinnigrade  (Seal);  C,  Digiti- 
grade  (Lion). 


most  marked  in  animals  of  great  speed,  as  the  horse. 
Some  walk  on  hoofs,  as  the  ox  (ungulate) ;  some  on  the 
toes,  as  the  cat  (digitigrade);  others  on  the  sole,  touch- 
ing the  ground  with  the  heel,  as  the  bear  (plantigrade). 
In  the  pinnigrade  seal,  half  of  the  fore  limb  is  buried 
under  the  skin,  and  the  hind  limbs  are  turned  backward 
to  form  a  fin  with  the  tail.  The  normal  number  of  toes 
is  five ;  but  some  may  be  wanting,  so  that  we  have  one- 
toed  animals  (as  horse),  two-toed  (as  ox),  three-toed  (as 
rhinoceros),  four-toed  (as  hippopotamus),  and  five-toed 


374 


COMPARATIVE   ZOOLOGY 


(as  the  elephant).  The  horse  steps  on  what  corre- 
sponds to  the  nail  of  the  middle  finger ;  and  its  swiftness 
is  conditioned  on  the  solidity  of  the  extremities  of  the 


FIG.  326. — Feet  of  Hoofed  Mammals:  A,  Elephant;  B,  Hippopotamus;  C,  Rhinoceros; 
Z>,  Ox;  E,  Horse,  a,  astragalus;  cl,  calcaneum,  or  heel;  s,  naviculare;  bt  cu- 
boides;  ce,  ci,  cm,  cuneiform  bones;  the  numbers  indicate  the  digits  in  use. 

limbs.     Horses  of  the  greatest  speed  have  the  shoulder 
joints  directed  at  a  considerable  angle  with  the  arm. 

The  order  in  which  the  legs  of  quadrupeds  succeed' 
each  other  determines  the  various  modes  of  progression, 
called  the  walk,  trot,  gallop,  and  leap.  Many,  as  the 
horse,  have  all  these  movements ;  while  some  only  leap, 
as  the  frog  and  kangaroo.  In  leaping  animals,  the  hind 
limbs  are  extraordinarily  developed.  In  many  mammals, 
like  the  squirrel,  cat,  and  dog,  the  fore  legs  are  used  for 
prehension  as  well  as  locomotion.  Monkeys  use  all  four, 
and  also  the  tail,  for  locomotion  and  prehension,  keeping 
a  horizontal  attitude ;  while  the  apes,  half  erect,  as  if 
they  were  half  quadruped,  half  biped,  go  shambling 
along,  touching  the  ground  with  the  knuckles  of  one 
hand  and  then  of  the  other.  In  descending  the  scale, 
from  the  most  anthropoid  ape  to  the  true  quadruped,  we 
find  the  center  of  gravity  placed  increasingly  higher  up 


HOW  ANIMALS   MOVE 


375 


—  that  is,  farther  forward.     Birds  and  men  are  the  only 
true  bipeds,  the  former  standing  on  their  toes,  the  latter 


FIG.  327. —  Muscles  of  the  Human  Leg: 
sartorzus,  or  "  tailor's  muscle,"  the 
longest  muscle  in  the  body,  flexes  the 
leg  upon  the  thigh:  rectus  femoris 
and  i>astus  externus  and  internus  ex- 
tend the  leg,  maintaining  an  erect 
posture;  gastrocnemius,  or  "calf," 
used  chiefly  in  walking,  for  raising  the 
heel.  Another  layer  underlies  these 
superficial  muscles. 


FIG.  328.  —  Muscles  of  an  Insect's  Leg 
(Melolontha  vulgaris):  a,  flexor,  and 
b,  extensor,  of  tibia;  c,  flexor  of  foot; 
</,  accessory  muscle ;  e ,  extensor  of 
claw;  f,  extensor  of  tarsus.  The 
joints  are  restricted  to  movements 
in  one  plane;  and  therefore  the  mus- 
cles are  simply  flexors  and  extensors. 
All  the  muscles  are  within  the  skele- 


on the  soles  of  the  feet.  Terrestial  birds  walk  and  run ; 
while  birds  of  flight  usually  hop.  The  ostrich  can  for  a 
time  outrun  the  Arabian  horse ;  and  the  speed  of  the 
cassowary  exceeds  that  of  the  swiftest  greyhound. 


CHAPTER   XXI* 

THE   NERVOUS   SYSTEM 

Nerve  Tissue  exists  in  the  form  of  cells  and  fibers, 
the  latter  being  prolongations  of  the  bodies  of  the  former. 
Where  the  cells  predominate,  nerve  tissue  is  grayish. 
Such  accumulations  are  called  ganglia,  or  nerve  centers, 
and  these  alone  originate  nervous  force ;  the  fibers 
are  generally  white,  and  arranged  in  bundles,  called 


FIG.  329. — Nerve  cell 
from  cerebral  cortex  : 
«,  nerve  process. 


FIG.  330.  —  Diagram  of  Nervous  Sys- 
tem of  Starfish:  r,  nervous  ring 
around  mouth;  n,  radial  nerves  to 
each  arm,  ending  in  the  eye. 


nerves,  which  serve  only  as  conductors.  Most  nerves 
contain  two  kinds  of  fibers,  like  in  structure,  but 
each  having  its  distinct  office :  one  carries  impres- 
sions received  from  the  external  world  to  the  gray 
centers,  and  hence  is  called  an  afferent,  or  sensory, 
nerve ;  the  other  conducts  an  influence  generated  in 
the  center  to  the  muscles,  in  obedience  to  which  they 

*  See  Appendix. 
376 


THE   NERVOUS    SYSTEM 


377 


contract,  and    hence  it  is  called   an  efferent,  or  motor, 

nerve.     Thus,  when   the  finger  is   pricked  with  a  pin, 

afferent  nerve  fibers  convey  the  impression  to  the  center, 

the  spinal  cord,  which  immediately  transmits  an  order 

by  efferent  fibers  to  the  muscles  of  the  hand  to  contract. 

If  the  former  are  cut,  sensation  is  lost,  but  voluntary 

motion  remains ;  if  the  latter  are  cut,  the  animal  loses 

all   control    over   the    muscles, 

although  sensibility  is  perfect; 

if  both  are  cut,  the  animal  is 

said  to  be  paralyzed  in  the  parts 

which    these     nerves     control. 

The  nerve  fibers  are  connected 

with  nerve  cells  in  the  central 

organs,  and  at  the  outer  ends 

are  connected  with  the  muscular 

fibers,  or  with  various  sensory 

end  organs  in  the  skin  or  other 

parts  of  the  body.     The  nature 

of  nerve  force  is  not  known. 

AS  tO  the  Velocity  Of    a   nerVOUS  FlG-    33^-  Nervous    System    of   a 

•*  Mollusk  (the  Gastropod  Aplysta) : 

impulse,  We    knOW   it    is  far    leSS  a,  anterior  ganglion;  c,  cephalic; 

,1            ,1              £      i       ,     .    .,               1.     i   ,  /,  lateral ;  g,  abdominal. 

than  that  or  electricity  or  light, 

and  that  it  is  more  rapid  in  warm-blooded  than  in 
cold-blooded  animals,  being  faster  in  man  than  in  the 
frog.  In  the  latter  it  averages  about  85  feet  per  second, 
the  former  over  100  feet. 

The  very  lowest  animals,  like  the  amoeba  and  In- 
fusoria, have  no  nerves,  although  their  protoplasm  has 
a  general  sensibility.  The  hydra  has  certain  cells  which 
are,  perhaps,  partly  nervous  and  partly  muscular  in 
function.  The  jellyfish  has  a  nervous  system,  consist- 
ing of  a  network  of  threads  and  ganglia  scattered  all 
over  its  disk.  We  should  look  for  a  definite  system  of 
ganglia  and  nerves  only  in  those  animals  which  possess 


378 


COMPARATIVE   ZOOLOGY 


a  definite  muscular  structure,  and  show  definitely  coordi- 
nated muscular  movements.  In  the  starfish  we  detect 
the  first  clear  specimen  of  such  a  sys- 
tem. It  consists  of  a  ring  around  the 
mouth,  made  of  five  ganglia  of  equal 
size,  with  radiating  nerves.  The  mol- 
lusks  are  distinguished  by  an  irregu- 
larly scattered  nervous  system.  The 
clam  has  three  main  pairs  of  connected 
/  ganglia  —  one  near  the 
mouth,  one  in  the  foot,  and 
the  third  in  the  posterior 
region,  near  the  syphons. 
In  the  snail,  these  are 
united  into  a  ring  around 
the  gullet,  and  there  are 

FIG.  332.  —  Nervous  Sys-  -  ,.  , 

tem  of  ciam;  , ,  cere-    other  ganglia  scattered 

bral  ganglion  ;  /,  pedal       thrOUgh  the  body.        The 
ganglia  ;      ps,      pane-  J 

tosplanchnic  ganglia  ; 
c' ,  cerebral  commis- 
sure ;  p',  commissure 
from  cerebral  to  pedal 
ganglia  ;  ps' ,  commis- 
sure from  cerebral  to 
parietosplanchnic 
ganglia ;  es,  esopha- 
gus. 


same  is  true  of  the  cuttle- 
fish, where  the  brain  is 
partly  inclosed  in  a  carti- 
laginous box  (Fig.  348). 
In  the  simpler  worms 
there  is  but  a  single 
ganglion  or  a  single  pair.  The  earthworm 
has  a  pair  of  brain  ganglia  lying  above  the 
gullet,  and  connected  by  two  cords  with  a 
ventral  chain  of  ganglia  —  one  pair,  appar- 
ently a  single  ganglion,  for  each  segment, 
In  the  lower  arthropods,  such  as  Crustacea, 
centipedes,  and  larval  insects,  the  arrange- 
ment is  substantially  the  same.  In  higher 
insects  and  Crustacea,  many  of  the  ganglia  are  fused  to- 
gether in  the  head  and  thorax,  indicating  a  concentration 
of  organs  for  sensation  and  locomotion. 


pillar  (Sphinx  li- 
gustrt):  the  first 
is  the  cephalic,  or 
head,  ganglion. 


THE   NERVOUS   SYSTEM 


379 


In  vertebrates,  the  nervous 
developed,  more  complex,  and 
more  concentrated  than  in  the 
lower  forms.  In  fact,  there  are 
some  parts,  as  the  brain,  to 
which  we  find  nothing  homolo- 
gous in  the  invertebrates  ;  and 
while  the  actions  of  the  latter 
are  mainly,  if  not  wholly,  auto- 
matic, those  of  backboned  ani- 
mals are  largely  voluntary.  Its 
position,  moreover,  is  peculiar, 
the  great  mass  of  the  nervous 
matter  being  accumulated  on 
the  dorsal  side,  and  inclosed 
by  the  neural  arches  of  the 
skeleton. 

The  brain  and  spinal  cord 
lie  in  the  cavity  of  the  skull 
and  spinal  column,  wrapped  in 
three  membranes.  Each  con- 
sists of  gray  and  white  nervous 
matter;  but  in  the  brain  the 
gray  is  on  the  outside,  and  the 
white  within ;  while  the  white 
of  the  spinal  cord  is  external, 
and  the  gray  internal.  Both 
are  double,  a  deep  fissure  run- 
ning from  the  forehead  back- 
ward, dividing  the  brain  into 
two  hemispheres,  and  the  spinal 
cord  resembling  two  columns 
welded  together ;  even  the 
nerves  come  forth  in  pairs  to 
the  right  and  left.  The  brain  is 


system   is  more  highly 


FIG.  334.  —  Human  Brain  and  Spinal 
Cord,  seen  from  below,  about  one- 
tenth  natural  size  ;  «,  right  hemi- 
sphere of  cerebrum;  b,  anterior 
lobe;  c,  middle  lobe;  d,  medulla 
oblongata  ;  e,  cerebellum  ;  f,  first 
spinal  nerve  ;  g,  brachial  plexus 
.of  nerves  supplying  the  arms  ; 
h,  dorsal  nerves  ;  i,  lumbar  nerves ; 
k,  sacral  plexus  of  nerves  for  the 
limbs  ;  /,  cauda  equina:  the  figures 
indicate  the  twelve  pairs  of  cranial 
nerves,  of  which  i  is  olfactory, 
2  optic,  and  8  auditory. 


380  COMPARATIVE   ZOOLOGY 

the  organ  of  sensation  and  voluntary  motion ;  the  spinal 
cord  is  the  organ  of  involuntary  life  and  motion.  The 
brain,  above  the  medulla  oblongata,  may  be  removed, 
and  yet  the  animal,  though  it  cannot  feel,  will  live  for  a 
time,  showing  that  it  is  not  absolutely  essential  to  life ; 
in  fact,  the  brain  does  nothing  in  apoplexy  and  deep 
sleep.  All  of  the  cord,  except  that  part  containing  the 
centers  for  respiration  and  circulation,  may  also  be 
destroyed,  without  causing  immediate  death. 

The  Brain  is  that  part  of  the  nervous  system  con- 
tained in  the  skull.139  It  increases  in  size  and  com- 
plexity as  we  pass  from  the  fishes,  by  the  amphibians, 
reptiles,  and  birds,  to  mammals.  Thus  the  body  of  the 
cod  is  5000  times  heavier  than  its  brain  —  in  fact,  the 
brain  weighs  less  than  the  spinal  cord ;  while  in  man, 
the  brain,  compared  with  the  body,  is  as  I  to  36,  and  is 
40  times  heavier  than  the  spinal  cord.  The  brains  of 
the  cat  weigh  only  i  oz. ;  of  the  dog,  6  oz.  5^  dr. ;  and 
of  the  horse,  22  oz.  15  dr.  The  only  animals  whose 
brains  outweigh  man's  are  the  elephant  and  whale  — 
the  maximum  weight  of  the  elephant's  being  10  Ibs., 
and  of  the  whale's  5  Ibs. ;  while  the  human  does  not 
exceed  4  Ibs.  Yet  the  human  brain  is  heavier  in  pro- 
portion to  the  body.  But  quality  must  be  considered 
as  well  as  quantity,  else  the  donkey  will  outrank  the 
horse,  and  the  canary  bird,  man ;  for  their  brains  are 
relatively  heavier. 

The  main  parts  of  the  brain  are  the  cerebrum,  cerebel- 
lum, and  medulla  oblongata. 

The  cerebrum  is  a  mass  of  white  fibrous  matter  cov- 
ered by  a  layer  of  gray  cellular  matter.  In  the  lower 
vertebrates,  the  exterior  is  smooth ;  but  in  most  of  the 
mammals  it  is  convoluted,  or  folded,  to  increase  the 
amount  of  the  gray  surface.  The  convolutions  multiply 
and  deepen  as  we  ascend  the  scale  of  size  and  intelli- 


THE   NERVOUS    SYSTEM 


381 


gence,  being  very  complex  in  the  elephant  and  whale, 
monkey  and  man.  As  a  rule,  they  are  proportioned  to 
the  intelligence  of  the  animal;  yet  the  brains  of  the 
dog  and  horse  are  smoother  than  those  of  the  sheep 
and  donkey.  Evi- 
dently the  quality  of 
the  gray  matter  must 
be  taken  into  account. 
Save  in  the  bony 
fishes,  the  cerebrum 
is  the  largest  portion 
of  the  brain ;  in  man 
it  is  over  eight  times 
heavier  than  the  cere- 
bellum. 

The  cerebellum,  or 
"little  brain,"  lies  be- 
hind the  cerebrum, 
and,  like  it,  presents 
an  external  gray  layer, 
with  a  white  interior. 
In  mammals,  it  is  like- 
wise finely  convoluted, 
consisting  of  gray  and 
white  laminae,  and  is 
divided  into  two  lobes, 
or  hemispheres.  In 
the  rest  of  the  verte- 
brates, the  cerebellum 
is  nearly  or  quite 
smooth ;  and  in  the  lowest  fishes  it  is  merely  a  thin 
plate  of  nervous  matter.  In  many  vertebrates,  how- 
ever, it  is  larger  compared  with  the  cerebrum,  than 
in  man,  since  in  man  the  cerebrum  is  extraordinarily 
developed. 


FIG.  335.  —  Brain  of  the  Horse  —  upper  view,  one 
fourth  natural  size  ;  a,  medulla  oblongata  ; 
b,  lateral  and  middle  lobes  of  cerebellum;  c,  inter- 
lobular  fissure  ;  d,  cerebral  hemispheres;  e,  olfac- 
tory lobes. 


382 


COMPARATIVE  ZOOLOGY 


The  medulla  oblongata  is  the  connecting  link  between 
the  cerebrum  and  cerebellum  and  the  spinal  cord.  In 
structure,  it  resembles  the  spinal  cord  —  the  white 
matter  being  external  and  the  gray  internal.  The 
former  lies  beneath  or  behind  the  brain,  passing  through 
the  foramen  magnum  of  the  skull,  and  merging  im- 
perceptibly into  the  cord.  The  latter  is  a  continuous 
tract  of  gray  matter  inclosed  within  strands  of  white 
fibers.  It  usually  ends  in  the  lumbar  region  of  the 
vertebral  column,  but  in  fishes  it  reaches  to  the  end  of 
the  tail.  In  fishes,  amphibians,  and  reptiles,  the  cord 
outweighs  the  brain ;  in  birds  and  mammals,  the  brain 
is  heavier  than  the  cord.  In  man,  the  cord  weighs 
about  an  ounce  and  a  half. 

Besides  these  parts, 
there  are  also  the  olfac- 
tory and  the  optic  lobes, 
which  give  rise  respec- 
tively to  the  nerves  of 
smell  and  sight. 

The  parts  of  the  brain 
are  always  in  pairs ;  but 
in  relative  development 
^^  position  they  differ 
widely  in  the  several 
classes  of  vertebrates. 


FIG.  336.  —  Brain  of 
the  Perch,  upper 
view :  a,  cerebel- 

b,  optic 

c,  cere- 


lum  ; 
lobes 


Skr 


brum;  i,  olfactory      In     fishes     and     TCptileS, 
lobes'    p-    medulla      ^.r.  j     • 

theY  are  arranged  in  a    olfactory  lobes.   Hc 
horizontal  line;  in  birds 


FIG.  337. —  Brain  of  the 
Frog,  upper  view;  X  4: 
r,  olfactory  nerves :  Lol, 


cDerebral  hemispheres; 

Pn,  pineal  gland;  Fho 

and  mammals,  the  axis  of  the  spinal    and  shr,  third   and 

.     .  ,  fourth  ventricles  ;  Lop, 

cord  bends  to  nearly  a  right  angle  in     optic  lobes  -,  c  cerebei- 
passing  through  the  brain,  so  that  the     J0unmgatf  '*•  medulla  ob' 
lobes  no  longer  lie  in  a  straight  line. 
In  man,  the  fore  brain  is  so  developed  that  it  covers  all 
the  other  lobes.     In  looking  down  upon  the  brain  of  a 


THE   NERVOUS    SYSTEM 


383 


perch,  we  see  in  front  a  pair  of  olfactory  lobes  (which 
send  forth  the  nerves  of  smell),  behind  them  the  small 
cerebral  hemispheres,  then  the  large  optic  lobes  (near 
which  originate  the  nerves  of  sight),  and,  last  of  all,  the 
cerebellum.  Not  until  we  reach  man  and  the  apes  do 
we  find  the  cerebrum  so  highly  developed  as  to  overlap 
both  the  olfactory  lobes  in  front  and  the  cerebellum 
behind. 

Functions  of  the  Brain.  —  The  cerebrum  is  the  seat  of 
intelligence  and  will.  It  has  no  direct  communication 
with  the  outside  world,  receiving  its  consciousness  of 
external  objects  and  events  through  the  spinal  cord  and 
the  nerves  of  special  sense.140 

The  cerebellum  seems  to  preside  over  the  coordina- 
tion of  the  muscular  movements.  When  the  cerebellum 


FIG.  338.  —  A,  C,  upper  and  side  views  of  the  Brain  of  a  Lizard  ;  B,  D,  upper  and  side 
views  of  the  Brain  of  a  Turkey:  Olf,  olfactory  lobes  ;  Hmp,  cerebral  hemispheres  ; 
/>«,  pineal  gland  ;  Mb,  optic  lobes  of  the  middle  brain  ;  Cb,  cerebellum;  MO,  me- 
dulla oblongata  ;  it,  optic  nerves;  iv  and  vi,  nerves  for  the  muscles  of  the  eye  ; 
Py,  pituitary  body. 

is  removed,  the  animal  desires  to  execute  the  mandates 
of  the  will,  but  can  not ;  its  motions  are  irregular,  and  it 
acts  as  if  intoxicated.  It  is  usually  largest  in  animals 
capable  of  the  most  complicated  movements,  being 


COMPARATIVE   ZOOLOGY 


larger  in  the  ape  than  in  the  lion,  in  the  lion  than  in 
the  ox,  in  birds  than  in  reptiles.     The  cerebellum  of  the 


FIG.  339.  —  Brain  of  the  Cat  (Felis  do- 
mestica)  :  a,  medulla  oblongata; 
b,  cerebellum  ;  c,  cerebrum. 


FIG.  340.  —  Brain  of  the  Orang-outang 
upper  surface  ;  one  third  natural 
size. 


frog  is,  however,  smaller  than  that  of  fishes  (Figs.  336, 
337).  The  olfactory  and  optic  lobes  receive  the  mes- 
sages from  their  respective  nerves.  The  medulla 
oblongata  is  not  only  the  me- 
dium of  communication  between 
the  brain  and  the  spinal  cord, 


FIG.  341.  —  Human  Brain,  side  view: 
i,  medulla  oblongata  ;  3,  cerebellum  ; 
5,  frontal  convolutions  of  cerebrum. 


FIG.  342.  —  Human  Brain,  upper  view, 
one  fourth  natural  size :  i,  anterior 
lobes  ;  2,  posterior;  3,  great  median 
fissure. 


but  it  is  itself  a  nervous  center :   the  brain  above  and 
the  cord  below  may  be  removed  without  death  to  the 


THE   NERVOUS   SYSTEM 


385 


animal,  but  the  destruction  of  the  medulla  is  fatal.  Of 
the  twelve  pairs  of  nerves  issuing  from  the  contents  of 
the  skull  (encepJialoii),  ten  come  from  the  medulla  oblon- 
gata.  Among  these  are  the  nerves  of  hearing  and  taste, 
and  those  that  control  the  lungs  and  heart.  Respiration 
ceases  immediately  when  the  medulla  is  injured. 

The  spinal  cord  is  a  center  for  originating  involuntary 
actions,  and  is  also  a  conductor  —  transmitting  through 
its  cells  and  fibers  to 
the  brain  the  impres- 
sions received  by  the 
sensory  organs,  and 
taking  back  to  the 
motor  organs  the  im- 
pulses of  the  brain. 
In  man,  thirty-one 
pairs  of  nerves  arise 
from  the  cord  to  sup- 
ply the  whole  body, 
except  the  head. 
Each  nerve  has  an 
anterior  and  a  poste- 
rior root.  The  fibers 
of  the  former  go  to  the 
muscles,  and  carry 
the  impulses  which 
cause  muscular  con- 
traction (hence  called 
motor  fiber!) ;  those 
of  the  posterior  root 
convey  sensations 
from  the  exterior  to 

the  central  organs  (sensory).     The  fibers  leading  from 
the  brain  to  the  cord  cross  one  another  in  the  medulla 
oblongata,  so  that  if  the  right  cerebral  hemisphere  be 
DODGE'S  GEN.  ZOOL.  —  25 


Fig.  343.  —  Relation  of  the  Sympathetic  and  Spinal 
Nerves  :  c,  fissure  of  spinal  cord;  «,  anterior  root 
of  a  dorsal  spinal  nerve  ;  /,  posterior  root,  with 
its  ganglion  ;  a ' ,  anterior  branch  ;  /',  posterior 
branch  ;  s,  sympathetic  ;  f-,  its  double  junction  by 
nerve  filaments. 


386  COMPARATIVE  ZOOLOGY 

diseased,  the  left  side  of  the  body  loses  the  power  of 
voluntary  motion.' 

The  sympathetic  nervous  system  is  a  double  chain  of 
ganglia,  lying  along  the  sides  of  the  vertebral  column  in 
the  ventral  cavity.  From  these  ganglia  nerves  are 
given  off,  which,  instead  of  going  to  the  skin  and  mus- 
cles, like  the  spinal  nerves,  form  networks  about  those 
internal  organs  over  which  the  will  has  no  control,  as 
the  heart,  stomach,  and  intestines.  Apparently  their 
office  is  to  stimulate  these  organs  to  constant  activity, 
but  is  little  understood. 


I.    The  Senses 

Sensation  is  the  consciousness  of  impressions  on  the 
sensory  organs.  These  impressions  produce  some 
change  in  the  brain  ;  but  what  that  change  is,  is  a  dark- 
ness on  which  no  hypothesis  throws  light.  Obviously, 
we  feel  only  the  condition  of  our  nervous  system,  not 
the  objects  which  excite  that  condition.141 

All  animals  possess  a  general  sensibility  diffused  over 
the  greater  part  of  the  body.142  This  sensibility,  like  as- 
similation and  contractility,  is  one  of  the  primary  phys- 
iological properties  of  protoplasm.  But,  besides  this 
(save  in  the  very  lowest  forms),  they  are  endowed  with 
special  nerves  for  receiving  the  impressions  of  light, 
sound,  etc.  These  nerves  of  sense,  as  they  are  called, 
although  structurally  alike,  transmit  different  sensations: 
thus,  the  ear  can  not  recognize  light,  and  the  eye  can  not 
distinguish  sounds.  In  the  vertebrates,  the  organs  of 
sight,  hearing,  and  smell  are  situated  in  pairs  on  each 
side  of  tire  head  ;  that  of  taste,  in  the  mucous  membrane 
covering  the  tongue  ;  while  the  sense  of  touch  and  that 
of  temperature  are  diffused  over  the  skin,  including  the 
mucous  membrane  of  the  mouth,  throat,  and  nose. 


THE   NERVOUS    SYSTEM 


387 


Sight  and  hearing  are  stimulated,  each  by  one  agent 
only ;  while  touch,  taste,  and  smell  may  be  excited  by 
various  substances.  The  agents  awakening  sight,  hear- 
ing, touch,  and  the  sense  of  temperature  are  physical ; 
those  causing  taste  and  smell  are  chemical.  Animals 
differ  widely  in  the  numbers  and  keenness  of  their 
senses.  But  there  is  no  sense  in  any  one  which  does 
not  exist  in  some  other. 

Touch  is  the  simplest  and  the  most  general  sense ;  no 
animal  is  without  it,  at  least  in  the  form  of  general  sen- 
sibility. It  is  likewise  the 
most  positive  and  certain 
of  the  senses.  In  the  sea 
anemone,  snail,  and  in- 
sect, it  is  most  acute  in 
the  " feelers"  (tentacles, 
horns,  and  antennae) ;  M3 
in  the  oyster,  the  edge 
of  the  mantle  is  most 
sensitive ;  in  fishes,  the 
lips ;  in  snakes,  the  tongue ; 
in  birds,  the  beak  and 
under  side  of  the  toes ;  in  quadrupeds,  the  lips  and 
tongue ;  and  in  monkeys  and  man,  the  lips  and  the  tips 
of  the  tongue  and  fingers.  In  the  most  sensitive  parts 
of  birds  and  mammals,  the  true  skin  is  raised  up  into 
multitudes  of  minute  elevations, 
called  papilla,  containing  loops  of 
capillaries  and  nerve  filaments.  At 
the  ends  of  the  latter  are  the  es- 
sential organs  of  touch,  the  tactile 
corpuscles  and  the  touch  cells. 
There  is  a  correspondence  between 
the  *  delicacy  of  touch  and  the  development  of  intelli- 
gence. The  cat  and  dog  are  more  sagacious  than 


FIG.  344  — Antennae  of  various  Insects 
(magnified). 


FIG.  345.  —  Papillae  of  Human 
Palm,  x  35,  the  cuticle  being 
removed. 


388 


COMPARATIVE   ZOOLOGY 


hoofed  animals.  The  elephant  and  parrot  are  remark- 
ably intelligent,  and  are  as  celebrated  for  their  tactual 
power. 

Taste  is  more  refined  than  touch,  since  it  gives  a 
knowledge  of  properties  which  can  not  be  felt.  It  is 
always  placed  at  the  entrance  to  the  digestive  canal, 
as  its  chief  purpose  is  to  guide  animals  in  their  choice 
of  food.  Special  organs  of  taste  have  been  detected  in 
only  a  few  of  the  invertebrates,  though  all  seem  to 
exercise  a  faculty  in  selecting  their  food.  Even  in  fishes, 
amphibians,  reptiles,  and  birds  this  sense  is  very  obtuse, 
for  they  bolt  their  food.  But  the  higher  vertebrates 
have  it  well  developed.  If  is  confined  to  the  tongue, 
and  is  most  delicate  at  the  root.144  A  state  of  solution 
and  an  actual  contact  of  the  fluid  are  necessary  condi- 
tions. 

Smell  is  the  perception  of  odors,  i.e.,  certain  substances 
in  the  gaseous  or  volatile  state.    Many  invertebrates  have 
this  sense :  snails,  e.g.,  seem  to  be  guided  to  their  food 
by  its  scent,  and  flies  soon  find  a 
piece  of   meat.      In   the   latter   the 
organ   is    probably    located    on   the 
antennae.     In  vertebrates,  it  is  placed 
at  the   entrance  to  the   respiratory 
tube,  in  the  upper  region  of  the  nose. 
There   the   olfactory  nerves/which 
cavity-  issue  from  the  olfactory  lobe  of  the 

brain,  and  pass  through  the  ethmoid  bone,  or  roof  of  the 
nasal  cavity,  are  distributed  over  a  moist  mucous  mem- 
brane. The  odorous  substance,  in  a  gaseous  or  finely 
divided  state,  is  dissolved  in  the  mucus  covering  this 
membrane.  In  fishes  and  reptiles  generally,  this  organ 
is  feebly  developed ;  sharks,  however,  gather  from  a 
great  distance  around  a  carcass.  In  the  porpoises<  and 
whales  it  is  nearly  or  entirely  wanting.  Among  birds, 


THE    NERVOUS    SYSTEM 


389 


The   simplest 
fluid,  in  which 


waders  have  the  largest  olfactory  nerves.  It  is  most 
acute  in  the  carnivorous  quadrupeds,  and  in  some  wild 
herbivores,  as  the  deer.  In  man  it  is  less  delicate,  but 
has  a  wider  range  than  in  any  brute. 

Hearing  is  the   perception   of  sound, 
form  of  the  organ  is  a  sac  filled  with 
float  the   soft   and   delicate   ends  of 
the  auditory  nerve.    Usually  the  vibra- 
tions of  the  fluid  are  strengthened  by 
the  presence  of  minute  hard  granules, 
called    otoliths.      Most    invertebrates  _ 

FIG.  347.  —  Ear  of  a  Mol- 

have  no  more  complicated  apparatus     lusk   (Cycias\   greatly 

-   .  ,  .,    .  ,      ,  ,      .,  enlarged,     showing      the 

than  this  ;  and  it  is  probable  that  they 
can  distinguish  one  noise  from  an- 
other, but  neither  pitch  nor  intensity. 
The  organ  is  generally  double,  but  not  always  located  in 
the  head.  In  the  clam,  it  is  found  at  the  base  of  the 
foot ;  some  grasshoppers  have  it  in  the  fore  legs ;  and 
in  many  insects  it  is  on  the  wing.  Lobsters  and  crabs 
have  the  auditory  sacs  at  the  base  of  the  antennas.145 


otolith  in  the  center  of  a 
cavity  which  is  filled  with 
fluid,  and  whose  walls  are 
lined  by  ciliated  cells. 


FIG.  348.  —  Brain  and  Auditory  Apparatus  of  the  Cuttlefish:  a,  b,  brain;  c,  auditory 
apparatus;  d,  the  cavity  in  which  it  is  lodged  ;  e,f,g,  eyes  ;  i,  2,  3,  otoliths. 

A   complex   organ  of  hearing,    located  in  the  head, 
exists  in  all  vertebrates,   save  the  very  lowest  fishes. 


390 


COMPARATIVE  ZOOLOGY 


As  complete  in  man,  it  consists  of  the  following  parts : 
i.  The  external  ear  (which  is  peculiar  to  mammals146); 
the  auditory  canal,  about  an  inch  long,  lined  with  hairs 
and  a  waxy  secretion,  and  closed  at  the  bottom  by  a 
membrane,  called  tympanum,  or  "  drum  of  the  ear." 

2.  The  middle  ear,  con- 
taining three  little  bones 
(the     smallest    in     the 
body),    malleus,    incus, 
and    stapes,    articulated 
together.      The    cavity 
communicates  with  the 
external  air   by  means 
of  the  Eustachian  tube, 

FIG.  349-  -  Section  of  Human  Ear  :  a,  external     which  Opens  at  the  back 

part     of     the     mouth. 

3.  The  internal  ear,  or 
labyrinth,   an    irregular 
cavity  in  the  solid  part 

of  the  temporal  bone,  and  separated  from  the  middle 
ear  by  a  bony  partition,  which  is  perforated  by  two 
small  holes.  -  The  labyrinth  consists  of  the  vestibule,  or 
entrance ;  the  semicircular  canals  or  tubes ;  and  the 
cochlea,  or  spiral  canal.  While  the  other  parts  are  full 
of  air,  the  labyrinth  is  filled  with  a  liquid,  and  in  this 
are  the  ends  of  the  auditory  nerve.  The  vibrations  of 
the  air,  collected  by  the  external  ear,  are  concentrated 
upon  the  tympanum,  and  thence  transmitted  through 
the  chain  of  little  bones  to  the  fluid  in  the  labyrinth. 

The  essential  organ  of  hearing  is  the  labyrinth,  which 
is,  substantially,  a  bag  filled  with  fluid  and  nerve  fila- 
ments. Fishes  generally  have  but  little  more.  In 
amphibians  and  reptiles  there  are  added  a  tympanum,  a 
single  bone,  connecting  this  with  the  internal  ear,  the 
cochlea,  and  the  Eustachian  tube,  the  tympanum  being 


ear,  with  auditory  canal  ;  b,  tympanic  cavity 
containing  the  three  bones;  c,  hammer,  and  its 
three  muscles,  d,  e,f;  g,  tympanic  membrane, 
or  head  of  the  drum  ;  h,  Eustachian  tube  lead- 
ing to  the  pharynx  ;  z,  labyrinth,  with  semi- 
circular canals  and  cochlea  visible. 


THE   NERVOUS    SYSTEM 


391 


external.  Birds  have,  besides,  an  auditory  passage,  open- 
ing on  a  level  with  the  surface  of  the  head,  and  sur- 
rounded by  a  circle  of  feathers.  Most  mammals  have 
an  external  ear. 

Sight  is  the  perception  of  light.147  In  all  animals  it 
depends  upon  the  peculiar  sensitiveness  of  the  optic 
organ  to  the  luminous  vibrations.  In  vertebrates  the 
optic  nerve  comes  from  the  middle  mass  of  the  brain, 
in  invertebrates  it  is  derived  from  a  ganglion.  Many 
animals  are  utterly  destitute  of  visual  organs,  as  the 
Protozoa,  and  the  lower  radiates  and  mollusks,  besides 
intestinal  worms  and  the  blind  fishes  and  many  cave- 
animals  ;  but  the  protozoan  Euglena  has  a  red  pigment 
spot  which  is  probably  affected  by  light  waves  in  a 
manner  different  from  that  in  which  the  rest  of  the  body 
is  influenced.  The  eyes  of  the  starfish  are  at  the  tips 
of  its  arms  or  rays.  Those  of  the  sea  urchin  form  a  ring 
at  the  dorsal  pole  of  the  body.  Around  the  margin  of 
the  jellyfish  are  colored  spots,  supposed  to  be  rudimen- 
tary eyes  ;  but,  as  a  lens  is  wanting,  there  is  no  image ; 
so  that  the  creature  can  merely  distinguish  light  from 
darkness  and  color  without  form.  Such  an  eye  is 
nothing  but  a  collection  of  pigment  granules  on  the 
expansion  of  a  nervous  thread,  and  the 
perception  of  light  is  probably  the  sensa- 
tion of  warmth,  the  pigment  absorbing 
the  rays  and  converting  them  into  heat. 

Going  higher,  we  find  a  lens  introduced, 
forming  a  distinct  image.     The  snail,  for  FIG.  35o.  — Eye  of 

,  ,  .          ,  11      i        Pecten,    much    en- 

example,  has  two  simple  eyes,  called  iarged:  m>  mantle; 
ocelli,  mounted  on  the  tip  of  its  long  !JensjJjf ™ 
tentacles,  each  consisting  of  a  globular  nerve- 
lens,148  with  a  transparent  skin  (cornea)  in  front,  and 
a  colored  membrane  (choroid)  and  a  nervous  network 
(retina)  behind.  The  scallop  (Pecten)  has  such  eyes  in 


392 


COMPARATIVE   ZOOLOGY 


the  edge  of  its  mantle  (Fig.  350).     Such  organs  are  the 
only  eyes  possessed  by  myriapods,  spiders,  scorpions, 

and  caterpillars.  Adult 
insects  usually  have  three 
ocelli  on  the  top  of  the 
head.  But  the  proper 
visual  organs  of  lobsters, 
crabs,  and  insects  are  two 
compound  eyes,  perched 
on  pedestals,  or  fixed  on 
the  sides  of  the  head. 
They  consist  of  an  im- 
mense number  of  ocelli 
pressed  together  so  that 
they  take  an  angular 

FIG.  351.  —  Head  of  a  Snail  bisected,  showing  form  foUT-Sided       in 

structure  of  tentacles:  a,  right  inferior  tentacle  .         .  ,      ,       . 

retracted  within  the  body;    b,   right  superior  CrUStaCCa,      SIX-Slded      in 

tentacle  fully  protruded  ;  c,  left  superior  ten-  •  .-j-%1  r 

tacle   partially   inverted;  rf,  left   inferior   ten-  IHSCCtS.       They  form   tWO 

tacle;/,  optic  nerve  ;  g    retractor  muscle;  roim(Je(J       protuberances 
ft,  optic  nerve  in  loose  folds;  /,  retractor  muscle 

of  head  ;  k,  nerve  and  muscle  of  left  inferior  Variously      Colored  

tentacle;  /,  m,  nervous  collar.  .  . 

white,  yellow,  red,  green, 

purple,  brown,  or  black.  Under  the  microscope,  the 
surface  is  seen  to  be  divided  into  a  host  of  facets,149 
each  being  an  ocellus  complete  in  itself.  Each  cornea 
is  convex  on  one  side,  and  either  convex  or  flat  on  the 
other,  so  that  it  produces  a  focus  like 
a  lens.  Behind  the  cornea,  or  lens,  is 
the  pigment,  having  a  minute  aperture 
or  "  pupil."  Next  is  a  conical  tube 
—  one  for  each  facet  —  with  sides  and 
bottom  lined  with  pigment.  These 
tubes  converge  to  the  optic  ganglion, 
the  fibers  of  which  pass  through  the  FIG.  352. -Head  of  the  Bee. 

_  showing  compound  eyes, 

tubes     tO     the     COrnea.150         Vision      by        the  three  ocelli,  or  stem- 
,  ,  .  mata,    and    the    antennae 

such  a  compound  eye  is  not  a  mosaic  ;     (magnified). 


THE   NERVOUS    SYSTEM 


393 


but  each  ocellus  gives 
a  complete  image, 
although  a  different 
perspective  from  its 
neighbor.  The  mul- 
tiplied images  are  re- 
duced to  one  mental 
stereoscopic  picture, 
on  the  principle  of 
single  vision  in  our- 
selves. 

The  eyes  of  the 
cuttlefish  are  the 
largest  and  the  most 

perfect  among  mver-   FlG    353._EyeofaBeetle(A/*&/o«*A«):  A,-section; 
TheV      re-       a'  °P^C  ganglion  ;  b,  secondary  nerves  ;  c,  retina; 
d,  pigment  layer;  e,  proper  optic  nerves;  B,  group 

the     eyeS     Of       of  ocelli  (magnified);  f,  bulb  of  optic  nerve;  g,  layer 
•t   •     ,  •          i  •  of  pigment;  k,  vitreous  humor;  /,  cornea. 

higher     animals     in 

having  a  crystalline  lens  with  a  chamber  in  front  (open, 
however,  to  the  sea  water),  and  a  chamber  behind  it 

filled  with   "  vitre- 
ous humor." 

The  eye  of  ver- 
tebrates is  formed 
by  the  infolding  of 
the  skin  to  create 
a  lens,  and  an  out- 
growth of  the  brain 
to  make  a  sensitive 
layer ;  both  inclosed 

FIG.  354.  —  Section  of  Human  Eye :  a.  and  b,  upper  and     .  ,   . 

lower  lid;  c,  conjunctiva,  or  mucous  membrane,  lining    in  a  WJllte 
the  inner  surface  ;  d,  external  membrane ;  e,  sheath  of 
optic  nerve  ;  f,  g,  muscles  for  rolling  the  eye  up  or 
down;  ht  sclerotic;  i,  transparent  cornea;  j,  choroid;     of  tOUffh  tiSSUC  with 
k,  I,  ciliary  muscle  for  adjusting  the  eye  for  distance; 

m,  iris  and  pupil;  «, canal;  0,retina;  s,  vitreous  humor;     a  transparent  front, 
f,  crystalline  lens  ;  v,  anterior  chamber  ;  x.  posterior  , ,      ,        , 

chan;ber.  called   the   cornea. 


394 


COMPARATIVE   ZOOLOGY 


This  case  is  kept  in  shape  by  two  fluids  —  the  thin 
aqueous  humor  filling  the  cavity  just  behind  the  cornea, 

and  the  jelly  like 
vitreous  humor  oc- 
cupying the  larger 
posterior  chamber. 
Between  the  two 
humors  lies  the  dou- 
ble-convex crystal- 
line lens.  On  the 
front  face  of  the 
lens  is  a  contractile 
circularcurtain(mV), 
with  a  hole  in  the 
center  (pupil} ;  and 
lining  the  sclerotic 
coat  is  the  choroid 
membrane,  covered 
with  dark  •  pigment. 
The  optic  nerve,  en- 
tering at  the  back  of 
the  eye  through  the 
sclerotic  and  choroid 
coats,  expands  into 
the  transparent  re- 
tina, which  consists 
of  several  layers  — 
fibrous,  cellular,  and 

FIG.    355.  —  Section    of   the    Human    Retina,  x  400 

i,  internal  limiting  membrane  ;  2,  optic-nerve  fibers  granular.       The  mOSt 

3,    ganglion    cells;    4,    internal    molecular    layer  Q~nQ1V:vp    nqri-    ,'<,    f-Up 

5,  internal  granules  ;   6,   external  molecular  layer  benblUVC    part    Ib    LUC 

7,  external  granules  ;  8,  external  limiting  membrane  surf  aCC  IvinST  next  tO 
9,  layer  of  rods  and  cones;  10,  pigment  layer.  "Jo 

the   black   pigment. 

And  here  is  a  peculiarity  of  the  vertebrate  eye  :  the 
nerve  fibers,  entering  from  behind,  turn  back  and  look 
toward  the  bottom  of  the  eye,  so  that  vision  is  directed 


THE   NERVOUS   SYSTEM 


395 


backward  ;  while  invertebrate  vision  is  directly  forward. 
In  vertebrates  only,  the  optic  nerves  cross  each  other 
(decussate)  in  passing  from  the  brain  to  the  eyes  ;  so  that 
the  right  side  of  the  brain,  e.g.,  receives  the  impressions 
of  objects  on  the  left  side  of  the  body.151 

Generally,  the  eyes  of  vertebrates  are  on  opposite 
sides  of  the  head ;  but  in  the  flatfishes  both  are  on  the 
same  side.  Usually,  both  eyes  see  the  same  object  at 
once;  but  in  most  fishes  the  eyes  are  set  so  far  back, 
the  fields  of  vision  are  distinct.  The  cornea  may  be 
flat,  and  the  lens  globular,  as  in  fishes ;  or  the  cornea 
very  convex,  and  the  lens  flattened,  as  in  owls.  Purely 
aquatic  animals  have  neither  eyelids  nor  tears,  but  nearly 
all  others  (especially  birds)  have  three  lids.152  The  pupil 
is  usually  round;  but  it  may  be  rhomb-shaped,  as  in 
frogs ;  vertically  oval,  as  in  crocodiles  and  cats ;  or 
transversely  oval,  as  in  geese,  doves,  horses,  and  rumi- 
nants. Many  quadrupeds,  as  the  cat,  have  a  membrane 
(tapetum)  lining  the  bottom  of  the  eyeball,  with  a  brilliant 
metallic  luster,  usually  green  or  pearly ;  it  is  this  which 
makes  the  eyes  of  such  animals  luminous  in  the  dark. 

/ 

2.    Instinct  and  Intelligence 

The  simplest  form  of  nervous  excitement  is  mere 
sensation.  Above  this  we  have  sensation  awakening 
consciousness,  out  of  which  come  those  voluntary  activi- 
ties grouped  together  under  the  name  of  Instinct ;  and, 
finally,  Intelligence. 

The  lowest  forms  of  life  are  completely  mechanical, 
for  their  movements  seem  to  be  due  solely  to  their 
organization.  They  are  automatons,  or  creatures  of 
necessity.  In  the  higher  animals  certain  actions  are 
automatic,  as  breathing,  the  beating  of  the  heart,  the 
contractions  of  the  iris,  and  all  the  first  movements  of 


396 


COMPARATIVE   ZOOLOGY 


an  infant.153  But,  generally,  the  actions  of  animals  are 
not  the  result  of  mere  bodily  organization. 

The  inferior  orders  are  under  the  control  of  Instinct, 
i.e.,  an  apparently  untaught  ability  to  perform  actions 
which  are  useful  to  the  animal.154  They  seem  to  be 
born  with  a  measure  of  knowledge  and  skill  (as  man 
is  said  to  have  innate  ideas),  acquired  neither  by  reason 
nor  experiment.  For  what  could  have  led  bees  to 
imagine  that  by  feeding  a  worker  larva  with  royal  jelly, 
instead  of  beebread,  it  would  turn  out  a  queen  instead 
of  a  neuter?  In  this  case,  neither  the  habit  nor  the 
experience  could  be  inherited,  for  the  worker  bees  are 
sterile.  We  can  only  guess  that  the  discovery  has  been 
communicated  by  the  survivors  of  an  older  swarm. 
Uniformity  is  another  characteristic  feature  of  instinct. 
Different  individuals  of  the  same  species  execute  pre- 
cisely the  same  movements  under  like  circumstances. 
The  career  of  one  bee  is  the  career  of  another.  We  do 
not  find  one  clever  and  another  stupid.  Honeycombs 
are  built  now  as  they  were  before  the  Christian  era. 
The  creatures  of  pure  instinct  appear  to  be  tied  down, 
by  the  constitution  of  their  nervous  system,  to  one  line 
of  action,  from  which  they  can  not  spontaneously  depart. 
The  actions  vary  only  as  the  structure  changes.155  There 
is  a  wonderful  fitness  in  what  they  do,  but  there  is  no 
intentional  adaptation  of  means  to  ends. 

All  animals,  from  the  starfish  to  man,  are  guided  more 
or  less  by  instinct ;  but  the  best  examples  are  furnished 
by  the  insect  world,  especially  by  the  social  hymenop- 
ters  (ants,  bees,  and  wasps).  The  butterfly  carefully 
provides  for  its  young,  which  it  is  destined  never  to 
see ;  many  insects  feed  on  particular  species  of  plants, 
which  they  select  with  wonderful  sagacity  ;  and  monkeys 
avoid  poisonous  berries ;  bees  and  squirrels  store  up 
food  for  the  future ;  bees,  wasps,  and  spiders  construct 


THE   NERVOUS   SYSTEM 


with  marvelous  precision  ;  and  the  subterranean  cham- 
bers of  ants  and  the  dikes  of  the  beaver  show  engineer- 
ing skill  ;  while  salmon  go  from  the  ocean  up  the  rivers 
to  spawn  ;  and  birds  of  the  temperate  zones  migrate 
with  great  regularity. 

But  in  the  midst  of  this  automatism  there  are  the 
glimmerings  of  intelligence  and  free  will.  We  see  some 
evidence  of  choice  and  of  designed  adaptation.  Pure 
instinct  should  be  infallible.  Yet  we  notice  mistakes 
that  remind  us  of  mental  aberrations.  Bees  are  not  so 
economical  as  has  been  generally  supposed.  A  mathe- 
matician can  make  five  cells  with  less  wax  than  the  bee 
uses  for  four;  while  the  bumblebee  uses  three  times 
as  much  material  as  the  hive  bee.  An  exact  hexagonal 
cell  does  not  exist  in  nature.  Flies  lay  eggs  on  the  car- 
rion plant  because  it  happens  to  have  the  odor  of  putrid 
meat.  The  domesticated  beaver  will  build  a  dam  across 
its  apartment.  Birds  frequently  make  mistakes  in  the 
construction  and  location  of  their  nests.  In  fact,  the 
process  of  cheating  animals  relies  on  the  imperfection 
of  instinct.  Nor  are  the  actions  of  the  brute  creation 
always  perfectly  uniform;  and,  so  far  as  animals  con- 
form to  circumstances,  they  act  from  intelligence,  not 
instinct.  There  is  proof  that  some  animals  profit  by 
experience.  Birds  do  learn  to  make  their  nests  ;  and 
the  older  ones  build  the  best.  Trappers  know  well  that 
young  animals  are  more  easily  caught  than  old  ones. 
Birds  brought  up  from  the  egg,  in  cages,  do  not  make 
the  characteristic  nests  of  their  species  ;  nor  do  they 
have  the  same  song  peculiar  to  their  species,  if  they 
have  not  heard  it.  Chimney  swallows  certainly  built 
their  nests  differently  in  America  three  hundred  years 
ago.  A  bee  can  make  cells  of  another  shape,  for  it  some- 
times does  ;  its  actions,  therefore,  being  elective  and  con- 
ditional, are  in  a  measure  the  result  of  calculation. 


398  COMPARATIVE   ZOOLOGY 

The  mistakes  and  variations  of  instinct  are  indications 
that  animals  have  something  more  —  a  limited  range  of 
that  principle  of  Intelligence  so  luminous  in  man.  No 
precise  line  can  be  drawn  between  instinctive  and  intel- 
ligent acts ;  all  we  can  say  is,  there  is  more  freedom  of 
choice  in  the  latter  than  the  former ;  and  that  some  ani- 
mals are  most  instinctive,  others  most  intelligent.  Thus, 
we  speak  of  the  instinct  of  the  ant,  bee,  and  beaver,  and 
the  intelligence  of  the  elephant,  dog,  and  monkey.  In- 
stinct loses  its  peculiar  character  as  intelligence  becomes 
developed.  Ascending  from  the  worm  and  oyster  to 
the  bee,  we  see  the  movements  become  more  complex  in 
character  and  more  special  in  their  objects ;  but  instinct 
is  supreme.  Still  ascending,  we  observe  a  gradual  fad- 
ing away  of  the  instincts,  till  they  become  subordinate  to 
higher  faculties — will  and  reason.  We  can  predict  with 
considerable  certainty  the  actions  of  animals  guided  by 
pure  instinct;  but  in  proportion  as  they  possess  the 
power  of  adapting  means  to  ends,  the  more  variable  their 
actions.  Thus,  the  architecture  of  birds  is  not  so  uniform 
as  that  of  insects.156 

We  must  credit  brutes  with  a  certain  amount  of  obser- 
vation and  imitation,  curiosity  and  cunning,  memory  and 
reason.  Animals  have  been  seen  to  pause,  deliberate, 
or  experiment  and  resolve.  The  elephant  and  horse, 
dog  and  monkey,  particularly,  participate  in  the  rational 
nature  of  man,  up  to  a  certain  point.  Thinking  begins 
wherever  there  is  an  intentional  adaptation  of  means  to 
ends  ;  for  that  involves  the  comparison  and  combination 
of  ideas.  Animals  interchange  ideas  :  the  whine  of  a 
dog  at  the  door  on  a  cold  night  certainly  implies  that  he 
wants  to  be  let  in.  Bees  and  ants,  it  is  well  known,  con- 
fer by  touching  together  their  antennae.  All  the  higher 
animals,  too,  have  similar  emotions  : —  as  joy,  fear,  love, 
and  anger. 


THE   NERVOUS    SYSTEM 


399 


While  instinct  culminates  in  insects,  the  highest  devel- 
opment of  intelligence  is  presented  in  man.167  In  man 
only  does  instinct  cease  to  be  the  controlling  power. 
He  stands  alone  in  having  the  whole  of  his  organization 
conformed  to  the  demands  of  his  brain;  and  his  intelli- 
gent acts  are  characterized  by  the  capacity  for  unlimited 
progress.  The  brutes  can  be  improved  by  domestica- 
tion ;  but,  left  to  themselves,  they  soon  relapse  into  their 
original  wildness.  Civilized  man  also  goes  back  to 
savagery ;  yet  man  (though  not  all  men)  has  the  ambi- 
tion to  exalt  his  mental  and  moral  nature.  He  has  a 
soul,  or  conscious  relation  to  the  infinite,  which  leads 
him  to  aspire  after  a  lofty  ideal.  Only  he  can  form 
abstract  ideas.  And,  finally,  he  is  a  completely  self- 
determining  agent,  with  a  prominent  will  and  conscience 
—  the  highest  attribute  of  the  animal  creation.  In  all 
this,  man  differs  profoundly  from  the  lower  forms  of 
life. 

»  3.    The  Voices  of  Animals 

Most  aquatic  animals  are  mute.  Some  crabs  make 
noises  by  rubbing  their  fore  legs  against  their  carapace ; 
and  many  fishes  produce  noises  in  various  ways,  mostly 
by  means  of  the  swim  bladder.  Insects  are  the  inverte- 
brates which  make  the  most  noise.  Their  organs  are 
usually  external,  while  those  of  vertebrates  are  internal. 
Insects  of  rapid  flight  generally  make  the  most  noise. 
In  some  the  noise  is  produced  by  friction  (stridulation) ; 
in  others,  by  the  passage  of  air  through  the  spiracles 
(humming).  The  shrill  notes  of  crickets  and  grasshop- 
pers are  produced  by  rubbing  the  wings  against  each 
other,  or  against  the  thighs ;  but  the  cicada,  or  harvest 
fly,  has  a  special  apparatus  —  a  tense  membrane  on  the 
abdomen,  acted  upon  by  muscles.  The  buzzing  of  flies 
and  humming  of  bees  are  caused,  in  part,  by  the  vibra- 


400 


COMPARATIVE   ZOOLOGY 


tions  of  the  wings ;  but  the  true  voice  of  these  insects 
comes  from  the  spiracles  of  the  thorax. 

Snakes  and  lizards  have  no  vocal  cords,  and  can  only 
hiss.  Frogs  croak  158  and  crocodiles  roar,  and  the  huge 
tortoise  of  the  Galapagos  Islands  utters  a  hoarse,  bellow- 
ing noise. 

The  vocal  apparatus  in  birds  is  situated  at  the  lower 
end  of  the  trachea,  where  it  divides  into  the  two  bronchi.159 
It  consists  mainly  of  a  bony  drum,  with  a  cross  bone, 
having  a  vertical  membrane  attached  to  its  upper  edge. 
The  membrane  is  put  in  motion  by  currents  of  air  pass- 
ing on  either  side  of  it.  Five  pairs  of  muscles  (in  the 
songsters)  adjust  the  length  of  the  windpipe  to  the 
pitch  of  the  glottis.  The  various  notes  are  produced 
by  differences  in  the  blast  of  air,  as  well  as  by  changes 
in  the  tension  of  the  membrane.  The  range  of  notes 
is  commonly  within  an  octave.  Birds  of  the  same 
family  have  a  similar  voice.  All  the  parrots  have  a 
harsh  utterance;  geese  and  ducks  quack  ;  crows,  magpies, 
and  jays  caw ;  while  the  warblers  differ  in  the  quality, 
rather  than  the  kind,  of  note.160  The  parrot  and  mock- 
ing bird  use  the  tongue  in  imitating  human  sounds. 
Some  species  possess  great  compass  of  voice.  The  bell- 
bird  can  be  heard  nearly  three  miles ;  and  Livingstone 
said  he  could  distinguish  the  voices  of  the  ostrich  and 
the  lion  only  by  knowing  that  the  former  roars  by  day, 
and  the  latter  by  night. 

The  vocal  organ  of  mammals,  unlike  that  of  birds,  is 
in  the  upper  part  of  the  larynx.  It  consists  of  four 
cartilages,  of  which  the  largest  (the  thyroid)  produces  the 
prominence  in  the  human  throat  known  as  "  Adam's 
apple,"  and  two  elastic  bands,  called  "  vocal  cords," 
just  below  the  glottis,  or  upper  opening  of  the  wind- 
pipe. The  various  tones  are  determined  by  the  tension 
of  these  cords,  which  is  effected  by  the  raising  or  lower- 


THE   NERVOUS    SYSTEM 


401 


ing  of  the  thyroid  cartilage,  to  which  one  end  of  the 
cords  is  attached.  The  will  cannot  influence  the  con- 
traction of  the  vocalizing  muscles,  except  in  the  very 
act  of  vocalization'.  The  vocal  sounds  produced  by 
mammals  may  be  distinguished  into  the  ordinary  voice, 
the  cry,  and  the  song.  The  second  is  the  sound  made 
by  brutes.  The  whale,  porpoise,  armadillo,  ant-eater, 
porcupine,  and  giraffe  are  generally  silent.  The  bat's 
voice  is  probably  the  shrillest  sound  audible 
to  human  ears.  There  is  little  modulation 
in  brute  utterance.  The  opossum  purrs, 
the  sloth  and  kangaroo  moan,  the  hog 
grunts  or  squeals,  the  tapir  whistles,  the 
stag  bellows,  and  the  elephant  gives  a 
hoarse  trumpet  sound  from  its  trunk  and 
a  deep  groan  from  its  throat.  All  sheep  FIG.  356. -Human 
have  a  guttural  voice ;  all  the  ox  family  ^J™/  ™a£ 
low,  from  the  bison  to  the  musk  ox;  all  °f  the  hy°id 

.  -Hi  bone ;  e,  trachea ; 

the  horses  and  donkeys  neigh  ;  all  the  cats  /,  esophagus;  s, 
miau,  from  the  domestic  animal  to  the  lion ;  ^s10"15- 
all  the  bears  growl ;  and  all  the  canine"  family  —  fox, 
wolf,  and  dog  —  bark  and  howl.  The  howling  monkeys 
and  gorillas  have  a  large  cavity,  or  sac,  in  the  throat  for 
resonance,  enabling  them  to  utter  a  powerful  voice ;  and 
one  of  the  gibbon  apes  has  the  remarkable  power  of  emit- 
ting a  complete  octave  of  musical  notes.  The  human 
voice,  taking  the  male  and  female  together,  has  a  range 
of  nearly  four  octaves.  Man's  power  of  speech,  or  the 
utterance  of  articulate  sounds,  is  due  to  his  intellectual 
development  rather  than  to  any  structural  difference 
between  him  and  the  apes.  Song  is  produced  by  the 
vocal  cords,  speech  by  the  mouth. 


DODGE'S  GEN.  ZOOL.  —  26 


CHAPTER   XXII 

REPRODUCTION 

IT  is  a  fundamental  truth  that  every  living  organism 
has  had  its  origin  in  some  preexisting  organism.  The 
doctrine  of  "  spontaneous  generation,"  or  the  supposed 
origination  of  organized  structures  out  of  inorganic  par- 
ticles, or  out  of  dead  organic  matter,  has  not  yet  been 
sustained  by  facts. 

Reproduction  is  of  two  kinds  —  sexual  and  asexual. 
All  animals,  probably,  have  the  first  method,  while  a 
very  great  number  of  the  lower  forms  of  life  have  the 
latter  also. 

Of  asexual  reproduction  there  are  two  kinds  —  Self- 
division  (Fission}  and  Budding. 

Self -division,  the  simplest  mode  possible,  is  a  natural 
breaking-up  of  the  body  into  distinct  surviving  parts. 
This  process  is  sometimes  extraordinarily  rapid,  the 
increase  of  one  animalcule  (Paramecium)  being  com- 
puted at  268  millions  in  a  month.  It  may  be  either 
transverse  or  longitudinal.  Of  the  first  sort,  Fig.  10  is 
an  example  ;  of  the  latter,  Fig.  1 1 ,  a.  This  form  of  re- 
production is,  naturally,  confined  to  animals  whose  tis- 
sues and  organs  are  simple,  and  so  can  easily  bear 
division,  or  whose  parts  are  so  arranged  as  to  be  easily 
separable  without  serious  injury.  The  process  is  most 
common  in  Protozoa,  worms,  and  polyps. 

Budding  is  separated  by  no  sharp  line  from  self- 
division.  While  in  the  latter  a  part  of  the  organs  of 
the  parent  go  to  the  offspring,  in  the  former  one  or 

402 


REPRODUCTION 


403 


more  cells  of  the  original  animal  begin  to  develop  and 
multiply  so  as  to  grow  into  a  new  animal  like  the 
parent.  The  process  in  animals  is  quite  akin  to  the 
same  operation  in  plants.  The  buds  may  remain  per- 
manently attached  to  the  parent  stock,  thus  making 
a  colony,  as  in  corals  and  Bryozoa  {continuous  budding), 
or  they  may  be  detached  at  some  stage  of  growth  (dis- 
continuous budding).  This  separation  may  occur  when 
the  bud  is  grown  up,  as  in  hydra  (Fig.  18),  or  as  in  plant 
lice,  daphnias  (Fig.  56),  and  among  other  animals  the 
buds  may  be  internal,  becoming  detached  when  entirely 
undeveloped  and  externally  resembling  an  egg.  They 
differ,  however,  entirely  from  a  true  egg  in  developing 
directly,  without  fertilization. 

Sexual  Reproduction  requires  cells  of  two  kinds, 
usually  from  different  animals.  These  are  the  germ 
cell  or  egg,  and  the  sperm  cell.  The  embryo  is  devel- 
oped from  the  cells  which  are  formed  by  the  repeated 
divisions  of  the  ovum  which  take  place  as  a  result  of  its 
union  with  the  sperm  cell.161 

The  egg  consists  essentially  of  three  parts,  the 
germinal  vesicle,  the  yolk,  and  the  vitelline  membrane, 
which  surrounds  both  the  first.  It  is  ordinarily  globular 
in  shape.  Of  the  three  parts,  the  primary  one  is  the 
germinal  vesicle  —  a  particle  of  protoplasm.  The  yolk 
serves  as  food  for  this,  and  the  membrane  protects  both. 
When  a  great  mass  of  yolk  is  present  it  is  divisible  into 
two  parts  — formative  and  food  yolk.  The  latter  is  of 
a  more  oily  nature  than  the  former,  and  is  usually  not 
segmented  with  the  egg.  The  structure  of  the  hen's 
egg  is  more  complicated.  The  outside  shell  consists  of 
earthy  matter  (lime)  deposited  in  a  network  of  animal 
matter.  It  is  minutely  porous,  to  allow  the  passage  to 
and  fro  of  vapor  and  air.  Lining  the  shell  is  a  double 
membrane  (membrana  putaminis)  resembling  delicate 


404 


COMPARATIVE   ZOOLOGY 


tissue  paper.     At  the  larger  end  it  separates  to  inclose 
a  bubble  of  air  for  the  use  of  the  chick.     Next  comes 
the  albumen,  or  "  white,"  in  spirally  ar- 
ranged layers,  within  which  floats  the 
yolk.     The  yolk  is  prevented  from  mov- 
ing toward  either  end  of  the  egg  by 
two  twisted   cords    of    albumen,  called 
chalazce ;   yet  is  allowed  to  rise  toward 
with  inclosed  cyto-  one  side,  the  yolk  being  lighter  than  the 

plasm;     n,    nucleus,  J 

consisting  of  nuclear  albumen.  1  he  yolk  is  composed  or  oily 
gra^ui^substa'nceln  granules  (about  gio"  of  an  inch  in  diam- 
which  are  seen  a  Qfcr\  and  js  inclosed  in  a  sac,  called 

spherical      nucleolus  ' 

and  several  irregular  the  vitelline  membrane,  and  disposed  in 

masses       of       chro-  .        .  ...  . 

matin;  a,  attraction  concentric  layers,   like  a   set  of   vases 

centroesomrining    *    PlaCed  OIle  wlthm  the  °ther-        That  Part 

of  the  yolk  which  extends  from  the 
center  to  a  white  spot  (cicatricula)  on  the  outside  can  not 
be  hardened,  even  with  the  most  prolonged  boiling. 
The  cicatricula,  or  embryo  spot,  is  a  thin  disk  of  cellular 
structure,  in  which  the  new  life  first  appears.  This  was 
originally  a  simple  cell,  but  development  has  gone  some 


FIG.  358.  —  Longitudinal  section  of  .Hen's  Egg  before  incubation:  « ,  yolk,  showing  con- 
centric layers;  a  ,  its  semifluid  center,  consisting  of  a  white  granular  substance  — 
the  whole  yolk  is  inclosed  in  the  vitelline  membrane  ;  b,  inner  dense  part  of  the 
albumen;  b' ,  outer,  thinner  part;  c,  the  chalaza,  or  albumen,  twisted  by  the  revolu- 
tions of  the  yolk  ;  d,  double  shell  membrane,  split  at  the  large  end  to  form  the 
chamber,//  e,  the  shell;  h,  the  white  spot,  or  cicatricula. 


REPRODUCTION 


405 


way  before  the  egg  is  laid.  It  is  always  on  that  side 
which  naturally  turns  uppermost,  for  the  yolk  can  turn 
upon  its  axis ;  it  is,  therefore,  always  nearest  to  the 
external  air  and  to  the  hen's  body  —  two  conditions 
necessary  for  its  development.  There  is  another  reason 
for  this  polarity  of  the  egg :  the  lighter  and  more  deli- 
cate part  of  the  yolk  is  collected  in  its  upper  region, 
while  the  heavy,  oily  portion  remains  beneath. 

In  most  eggs  the  shell  and  albumen  are  wanting. 
When  the  albumen  is  present,  it  is  commonly  covered 
by  a  membrane  only.  In  sharks  the  envelope  is  horny ; 
and  in  crocodiles  is  calcareous,  as  in 
birds. 

The  egg  of  the  sponge  has  no  true 
vitelline  membrane,  and  is  not  unlike 
an  ordinary  amoeboid  cell.  An  egg 
is,  in  fact,  little  more  than  a  very  large 
cell,  of  which  the  germinal  vesicle  is 
the  nucleus. 

The  size  of  an  egg  depends  mainly  FIG.  359.  -  Egg  of  Sponge 
upon  the  quantity  of  yolk  it  contains ;  (magnifi<  i:  "• nucleus' 
and  to  this  is  proportioned  the  grade  of  development 
which  the  embryo  attains  when  it  leaves  the  egg.162  In 
the  eggs  of  the  starfishes,  worms,  insects,  mollusks  (ex- 
cept the  cuttlefishes),  many  amphibians,  and  mammals, 
the  yolk  is  very  minute  and  formative,  i.e.,  it  is  con- 
verted into  the  parts  of  the  future  embryo.  In  the 
eggs  of  lobsters,  crabs,  spiders,  cephalopods,  fishes, 
reptiles,  and  birds,  the  yolk  is  large  and  colored,  and 
consists  of  two  parts  —  the  formative,  or  germ  yolk, 
immediately  surrounding  the  germinal  vesicle ;  and  the 
nutritive,  or  food  yolk,  constituting  the  greater  part  of 
the  mass,  by  which  the  young  animal  in  its  egg  life  is 
nourished.  In  the  latter  case,  the  young  come  forth 
more  mature  than  when  the  food  yolk  is  wanting. 


406  COMPARATIVE  ZOOLOGY 

As  to  form,  eggs  are  oval  or  elliptical,  as  in  birds  and 
crocodiles ;  spherical,  as  in  turtles  and  wasps ;  cylindri- 
cal, as  in  bees  and  flies ;  or  shaped  like  a  handbarrow, 
with  tendrils  on  the  corners,  as  in  the  shark.  The  eggs 
of  some  very  low  forms  are  sculptured  or  covered  with 
hairs  or  prickles. 

The  number  of  eggs  varies  greatly  in  different  ani- 
mals, as  it  is  in  proportion  to  the  risks  during  develop- 
ment Thus,  the  eggs  of  aquatic  tribes,  being  unprotected 
by  the  parent,  and  being  largely  consumed  by  many 


FIG.  360.  —  Egg  of  a  Shark  (the  external  gills  of  the  embryo  are  not  represented). 

animals,  are  numerous  to  prevent  extinction.  The  spawn 
of  a  single  cod  contains  millions  of  eggs ;  that  of  the 
oyster,  6,000,000.  A  queen  bee,  during  the  five  years 
of  her  existence,  lays  about  a  million  eggs. 

Eggs  are  laid  one  by  one,  as  by  birds  ;  or  in  clusters, 
as  by  frogs,  fishes,  and  most  invertebrates.  The  spawn 
of  the  sea  snails  consists  of  vast  numbers  of  eggs 
adhering  together  in  masses,  or  in  sacs,  forming  long 
strings. 

As  a  rule,  the  higher  the  rank,  the  more  care  animals 
take  of  their  eggs  and  their  young,  and  the  higher  the 
temperature  needed  for  egg  development.  In  the  major- 
ity of  cases,  eggs  are  left  to  themselves.  The  fresh- 
water mussel  (Unid)  carries  them  within  its  gills,  and 


REPRODUCTION 


the  lobster  under  its  tail.  The  eggs  of  many  spiders 
are  enveloped  in  a  silken  cocoon,  which  the  mother 
guards  with  jealous  care.  Insects,  as  flies  and  moths, 
deposit  their  eggs  where  the  larva,  as  soon  as  born,  can 
procure  its  own  food.  Most  fishes  allow  their  spawn, 
or  roe,  to  float  in  the  water  ;  but  a  few  build  a  kind  of 
flat  nest  in  the  sand  or  mud,  hovering  over  the  eggs 
until  they  are  hatched  ;  while  the  Acara  of  the  Amazon 
carries  them  in  its  mouth.  The  amphibians,  generally, 
envelop  their  eggs  in  a  gelatinous  mass,  which  they 
leave  to  the  elements  ;  but  the  female  of  the  Surinam 
toad  carries  hers  on  her  back,  where  they  are  placed  by 
the  male.  The  great  Amazon  turtles  lay  their  eggs  in 
holes  two  feet  deep,  in  the  sand  ;  while  the  alligators 
simply  cover  theirs  with  a  few  leaves  and  sticks. 
Nearly  all  birds  build  nests,  those  of  the  perchers  being 
most  elaborate,  as  their  chicks  are  dependent  for  a 
time  on  the  parent.163  The  young  of  marsupials,  as  the 
kangaroo,  which  are  born  in  an  extremely  immature 
state,  are  nourished  in  a  pouch  outside  of  the  body. 
But  the  embryo  of  all  other  mammals  is  developed 
within  the  parent  to  a  more  perfect  condition,  by  means 
of  a  special  organ,  the  placenta.  It  is  a  general  law, 
that  animals  receiving  in  the  embryo  states  the  longest 
and  most  constant  parental  care  ultimately  attain  the 
highest  grade  of  development. 

The  Protozoa,  which  have  no  true  eggs,  have  a  sort 
of  reproduction  called  conjugation.  In  this  process  two 
individuals  unite  into  one  mass,  surround  themselves 
with  a  case,  in  which  they  divide  into  several  parts, 
each  portion  becoming  a  new  individual,  or  the  process 
may  be  followed  by  repeated  divisions  of  the  two 
individuals  which  separate  as  soon  as  the  process  is 
finished,  as  in  Paramecitim,  or  remain  fused  together,  as 
in  Vorticella. 


408  COMPARATIVE   ZOOLOGY 

The  sperm  cells  differ  from  the  egg  in  being  very 
small,  commonly  motile,  and  in  that  a  large  number  are 
usually  produced  from  a  single  primary  reproductive 
cell,  while  the  egg  represents  the  entire  primary  cell. 
The  union  of  the  sperm  cell  with  the  germinal  vesicle 
{fertilization)  is  the  first  step  in  development,  and  with- 
out it  the  egg  will  not  develop  normally. 


CHAPTER   XXIII* 

DEVELOPMENT 

Development  is  the  evolution  of  a  germ  into  a  com- 
plete organism.  The  study  of  the  changes  in  the 
developing  embryo  constitutes  the  science  of  Embry- 
ology ;  the  transformations  after  the  egg  life  are  called 
metamorphoses,  and  include  growth  and  repair. 

The  process  of  development  is  a  passage  from  the 
general  to  the  special,  from  the  simple  to  the  complex, 
from  the  homogeneous  to  the  heterogeneous,  by  a  series 
of  differentiations.  It  brings  out 
first  the  profounder  distinctions, 
and  afterward  those  more  external.  a  *  t>  mo  c 

„,  ,  .    i  FIG.    361.  —  Fertilization  and 

That     IS,     the     mOSt     essential     partS        segmentation  of  mammalian 

appear     first.         And     not     Only    does        ovum:    ™,  spermatozoon; 

J  n,    nucleus ;     nu,    nucleo- 

development  tend  to  make  the  sev-     lus ;  *,  z°n*  nuiiata ;  cit 

.  .  segmenting  cell. 

eral  organs  of  an  individual  more 
distinct  from  one  another,  but  also  the  individual  itself 
more  distinguished  from  other  individuals  and  from  the 
medium  in  which  it  lives.  With  advancing  develop- 
ment, the  animal,  as  a  rule,  acquires  a  more  specific, 
definite  form,  and  increases  in  weight  and  locomotive 
power.  Life  is  a  tendency  to  individuality. 

The  first  step  in  development,  after  fertilization,  is  the 
segmentation  of  the  egg,  by  a  process  of  self-division. 
In  the  simplest  form,  the  whole  yolk  divides  into  two 
parts  ;  these  again  divide  repeatedly,  making  four,  eight, 
sixteen,  etc.,  parts,  until  the  whole  yolk  is  subdivided 

*  See  Appendix. 
409 


4io 


COMPARATIVE   ZOOLOGY 


into  very  small  portions  (cells)  surrounding  a  central 
cavity.  This  stage  is  known  as  the  "  mulberry  mass," 
or  blastula  (Fig.  361,  c).  If  the  yolk  is  larger,  relatively 
to  the  germinal  vesicle,  the  process  of  division  may  go 
on  more  slowly  in  one  of  the  two  parts  of  the  egg,  first 
formed  ;  or  in  very  large  eggs,  like  those  of  birds  and 
cuttlefishes,  only  a  small  part  of  the  yolk  subdivides. 

In  some  form,  the  process  of  segmentation  is  found  in 
the  eggs  of  all  animals,  as  is  also  the  following  stage. 
This  step  is  the  differentiation  of  the  single  layer  of 
cells  into  two  parts,  one  for  the  body  wall,  the  other  for 
the  wall  of  the  digestive  tract.  In 
the  typical  examples,  this  '  is  ac- 
complished by  one  part  of  the 
wall  of  the  blastula  turning  in  so 
far  as  to  convert  the  blastula  into 
a  sort  of  double-walled  cup,  the 
gastrula  (Fig.  362).  One  half  of 

FIG.  362.  -  Diagram  of  Gastrula  \     f     *        ' 

of  a  worm  ($«£*#«):«,  prim-    the  wall  of  the  blastula  is  now  the 
outer  wall  of  the  germ,  the  other 


body  cavity  ;  en,  endoderm  ;        foalf  that    Qf    ^Q    digestive    Cavity  I 
ec,  ectoderm. 

the  original  blastula  cavity  is  now 

the  body  cavity,  the  new  cavity  formed  by  the  infold- 
ing is  the  stomach  and  its  opening  is  both  mouth  and 
vent  (Fig.  362).  Some  adult  animals  are  little  more 
than  such  a  sac.  Hydra  (Fig.  18),  for  instance,  is 
little  different  from  a  gastrula  with  tentacles,  and  one 
of  its  relatives  wants  even  these  additions. 

Ordinarily,  however,  development  goes  much  further. 
From  the  two  original  layers  arises,  in  various  ways,  a 
third  between  them,  making  the  three  primitive  germ 
layers  —  epiblast,  mesoblast,  and  hypoblast.  This  new 
layer  is  necessarily  in  the  primitive  body  cavity,  which 
it  may  fill  up  ;  or  usually  a  new  body  cavity  is  formed, 
in  different  ways  in  different  groups.  In  by  far.  the 


DEVELOPMENT  411 

great  majority  of  animals  the  digestive  tract  gets  a  new 
opening,  which  usually  becomes  the  mouth  ;  and  the  old 
mouth  may  close,  or  serve  only  the  functions  of  the 
vent.  From  this  point  the  development  of  each  group 
must  be  traced  in  detail. 

Development  of  a  Hen's  Egg.  —After  the  segmentation, 
the  germinal  disk  divides  into  two  layers,  between  which 
a  third  is  soon  formed.  The  upper  layer  (epiblast)  gives 
rise  to  the  epidermis,  brain,  spinal  cord,  retina,  crystalline 
lens,  and  internal  ear.  From  the  lower  layer  (hypoblast) 
is  formed  the  epithelium  of  the  digestive  canal.  From 
the  middle  layer  (mesoblast)  come  all  the  other  organs 
—  muscles,  bones,  blood  vessels,  etc.  The  mesoblast 


FIG.  363.  —  Transverse  vertical  sections  of  an  egg,  showing  progressive  stages  of  develop- 
ment: a,  notochord  ;  b,  medullary  furrow,  becoming  a  closed  canal  in  the  last. 

thickens  so  as  to  form  two  parallel  ridges  running  length- 
wise of  the  germ,  and  leaving  a  groove  between  them 
(medullary  furrow  and  ridges)™  The  ridges  gradually 
rise,  carrying  with  them  the  epiblast,  incline  toward  each 
other,  and  at  last  unite  along  the  back.  So  that  we 
have  a  tube  of  epiblast  surrounded  by  mesoblast,  which 
is  itself  covered  by  epiblast.  This  tube  becomes  the 
brain  and  spinal  cord,  whose  central  canal,  enlarging 
into  the  ventricles  of  the  brain,  tells  the  story  of  its 
original  formation.  Beneath  the  furrow,  a  delicate 
cartilaginous  thread  appears  (called  notochord} — the 
predecessor  of  the  backbone.  Meanwhile  the  mesoblast 
has  divided  into  two  layers,  except  in  the  middle  of  the 
animal,  beneath  the  spinal  cord,  and  in  the  head.  One 
of  these  layers  remains  attached  to  the  epiblast,  and 


4I2  COMPARATIVE  ZOOLOGY 

with  it  forms  the  body  wall;  the  other  bends  rapidly 
downward,  carrying  the  hypoblast  with  it,  and  forms 
the  wall  of  the  intestine.  The  space  thus  left  between 
the  layers  of  the  mesoblast  is  the  body  cavity.  At  the 
same  time,  the  margin  of  the  germ  extends  farther  and 
farther  over  the  yolk,  till  it  completely  incloses  it.  So 
that  now  we  see  two  cavities  —  a  small  one,  containing 
the  nervous  system  ;  and  a  larger  one  below,  for  the 
digestive  organs.  Presently,  numerous  rows  of  cor- 
puscles are  seen  on  the  middle  layer,  which  are  subse- 
quently inclosed,  forming  a  network  of  capillaries,  called 
the  vascular  area.  A  dark  spot  indicates  the  situation 
of  the  heart,  which  is  the  first  distinctly  bounded  cavity 

of    the    circulatory 
system.       It    is    a 

short     tube     ^ing 
lengthwise  just  be- 

FIG.  364.  —  Rudimentary  Hearts,   human:   i,  venous      hind   the    head,   with 
trunks;  a,  auricle;  3,  ventricle  ;  4,  bulbus  arteripsus. 


causing  the  blood  to  flow  backward  and  forward. 
The  tube  is  gradually  bent  together,  until  it  forms  a 
double  cavity,  resembling  the  heart  of  a  fish.  On  the 
fourth  day  of  incubation  partitions  begin  to  grow,  divid- 
ing the  cavities  into  the  right  and  left  auricles  and 
ventricles.  The  septum  between  the  auricles  is  the  last 
to  be  finished,  being  closed  the  moment  respiration 
begins.  The  blood  vessels  ramify  in  all  directions 
over  the  yolk,  absorbing  its  substance,  and  all  perform- 
ing the  same  office  ;  it  is  not  till  the  fourth  or  fifth  day 
that  arteries  can  be  distinguished  from  veins,  by  being 
thicker,  and  by  carrying  blood  only  from  the  heart.165 

The  embryo  lies  with  its  face,  or  ventral  surface, 
toward  the  yolk,  the  head  and  tail  curving  toward 
each  -other.  Around  the  embryo  on  all  sides  the  epi- 
blast  and  upper  layer  of  the  mesoblast  rise  like  a  hood 


DEVELOPMENT 


413 


over  the  back  of  the  embryo  till  they  form  a  closed  sac, 
called  the  amnion.  It  is  filled  with  a  thin  liquid,  which 
serves  to  protect  the  embryo.  Meanwhile,  another  im- 
portant organ  is  forming  on  the  other  side.  From  the 


FlG.  365.  —  Embryo  in  a  Hen's  Egg  during  the  first  five  days,  longitudinal  view  : 
A,  hypoblast  ;  B,  lower  layer  of  mesoblast;  C,  upper  layer  of  mesoblast  and  epiblast 
united,  in  the  last  figures  forming  the  amniotic  sac  ;  'D,  vitelline  membrane  ; 
e,  thickened  blastoderm,  the  first  rudiment  of  the  dorsal  part  (in  the  last  figure  it 
marks  the  place  of  the  lungs);  h,  heart;  a,  b,  its  two  chambers ;  c,  aortic  arches  ; 
m,  aorta;  /,  liver;  /,  allantois. 


COMPARATIVE   ZOOLOGY 


FIG.  366.  — The  hen's  egg 
at  the   end   of  the   ninth 


up  in  order  to  show  its 
shape  more  clearly  : 
an,  inner  or  true  am- 
nion ;  hm,  hyomandibu- 


hinder  portion  of  the  alimentary  canal  an  outgrowth  is 
formed  which  extends  beyond  the  wall  of  the  embryo 
—  ta  hm  proper  into  the  cavity  of  the  amnion, 

and  spreads  out  over  the  whole  inner 
surface  of  the  shell,  so  that  it  partly 
surrounds  both  embryo  and  inner 
layer  of  the  amnion  (amnion  proper). 
This  is  the  .allantois.  It  is  full  of 
blood  vessels,  and  it  serves  as  the 
embryo  naturally  lies  with  respiratorv  organ  until  the  chick 

its  left  side  on    the   yolk  J  & 

sac,  but  has  been  lifted  picks  the  shell  and  breathes  by  its 
lungs.166  The  chorion  is  the  outer- 
most part  of  the  allantois,  and  the 

iar  cleft;  sv,  air  chamber;    placenta  of  mammals  is  the  shaggy, 

ta,    allantois;  -wa,  white 

or     albumen ;    ys,     yolk      VaSCular  edge  of  the  chOROn. 

The  alimentary  canal  is  at  first  a 

straight  tube  closed  at  both  ends,  the  middle  being 
connected  with  the  yolk  sac.  As  it  grows  faster  than 
the  body,  it  is  thrown  into  a  spiral  coil ;  and  at  several 
points  it  dilates,  to  form  the  crop,  stomach,  gizzard,  etc. 
The  mouth  is  developed  from  an  infolding  of  the  skin. 
The  liver  is  an  outgrowth  from 
the  digestive  tube,  at  first  a  clus- 
ter of  cells,  then  of  follicles,  and 
finally  a  true  gland.  The  lungs 
are  developed  on  the  third  day 
as  a  minute  bud  from  the  upper 
part  of  the  alimentary  canal,  or 
pharynx.  As  they  grow  in  size,  FlG-  367-  -  view  of  embryo  with 

1  its  foetal  membrane  :    am,  am- 

they     paSS     from     a  SmOOth     tO     a        nion  proper ;  d,  dwindled  yolk 

, ,     |                      , .    .  sac ;  al,  allantois ;  al*,  subzonal 

Cellular    Condition.  membrane;  z,  villi ;  outside  the 

The  skeleton  at  the  beginning     S^^nStok^h^ 
consists,  like  the  notochord^  of  a     blast,  z'. 
cellular   material,   which    gradually   turns   to  cartilage. 
Then  minute  canals  containing  blood  vessels  arise,  and 


DEVELOPMENT 


415 


earthy  matter  (chiefly  phosphate  of  lime)  is  deposited 
between  the  cells.  The  primary  bone  thus  formed  is 
compact :  true  osseous  tissue,  with  canaliculi,  laminae, 
and  Haversian  canals,  is  the  result  of  subsequent  ab- 
sorption.167 Certain  bones,  as  those  of  the  face  and 
cranium,  are  not  preceded  by  cartilage,  but  by  connec- 
tive tissue ;  these  are  called  membrane  bones.  Ossi- 
fication, or  bone  making,  begins  at  numerous  distinct 
points,  called  centers  of  ossification  ;  and,  theoretically, 
every  center  stands  for  a  bone,  so  that  there  are  as  many 
bones  in  a  skeleton  as  centers  of  ossification.  But  the 
actual  number  in  the  adult  animal  is  much  smaller,  as 
many  of  the  centers  coalesce.168  The  development  of 
the  backbone  is  not  from  the  head  or  from  the  tail,  but 
from  a  central  point  midway  between ;  there  the  first 
vertebrae  appear,  and  from  there  they  multiply  forward 
and  backward. 

The  limbs  appear  as  buds  on  the  sides  of  the  body ; 
these  lengthen  and  expand  so  as  to  resemble  paddles  — 
the  wings  and  legs  looking  precisely  alike ;  and,  finally, 
they  are  divided  each  into  three  segments,  the  last  one 
subdividing  into  digits.  The  feathers  are  developed 
from  the  outside  cells  of  the  epidermis  :  first,  a  horny 
cone  is  formed,  which  elongates  and  spreads  out  into  a 
vane,  and  this  splits  up  into  barbs  and  barbules. 

The  muscle  fibers  are  formed  either  by  the  growth  in 
length  of  a  single  cell,  or  by  the  coalescence  of  a  row  of 
cells  ;  the  cell  wall  thus  produces  a  long  tube  —  the  sar- 
colemma  of  a  fiber  —  and  the  granular  contents  arrange 
themselves  into  linear  series,  to  make  fibrillae. 

Nervous  tissue  is  derived  from  the  multiplication  and 
union  of  embryo  cells.  The  white  fibers  at  first  resem- 
ble the  gray.  The  brain  and  spinal  marrow  are  devel- 
oped from  the  epiblastic  lining  of  the  medullary  furrow. 
Soon  the  brain,  by  two  constrictions,  divides  into  fore 


4I6  COMPARATIVE   ZOOLOGY 

brain,  mid  brain,  and  hind  brains  The  fore  brain  throws 
out  two  lateral  hemispheres  (cerebrum),  and  from  these 
protrude  forward  the  two  olfactory  lobes.  From  the 
mid  brain  grow  the  optic  lobes ;  and  the  hind  brain  is 
separated  into  cerebellum  and  medulla  oblongata.  The 
essential  parts  of  the  eye,  retina  and  crystalline  lens, 
are  developed,  the  former  as  a  cuplike  outgrowth  from 
the  fore  brain,  the  latter  as  an  ingrowth  of  the  epider- 
mis. An  infolding  of  the  epidermis  gives  rise  to  the 
essential  parts  of  the  inner  ear,  and  from  the  same  layer 
come  the  olfactory  rods  of  the  nose  and  the  taste  buds 
of  the  tongue.  So  that  the  central  nervous  system  and 
the  essential  parts  of  most  of  the  sense  organs  have  a 
common  origin. 

Modes  of  Development.  —  The  structure  and  embryol- 
ogy of  a  hen's  egg  exhibit  many  facts  which  are  common 
to  all  animals.  But  every  grand  division  of  the  animal 
kingdom  has  its  characteristic  method  of  developing. 

Protozoans  differ  from  all  higher  forms  in  having  no 
true  eggs. 

The  egg  of  the  hydroid,  after  segmentation,  becomes 
a  hollow,  pear-shaped  body,  covered  with  cilia.  Soon 
one  end  is  indented ;  then  the  indentation  deepens  until 
it  reaches  the  interior  and  forms  the  mouth.  The  ani- 
mal fastens  itself  by  the  other  end,  and  the  tentacles 
appear  as  buds.  In  the  sea  anemone,  the  stomach  is 
turned  in,  and  the  partitions  appear  in  pairs. 

In  the  oyster,  the  egg  segments  into  two  unequal 
parts,  one  of  which  gives  rise  to  the  digestive  tract  and 
its  derivatives,  while  from  the  smaller  part  originate  the 
skin,  gills,  and  shell.  It  is  soon  covered  with  cilia,  by 
whose  help  it  swims  about. 

The  embryo  of  an  insect  shows  from  the  first  a  right 
and  left  side ;  but  the  first  indication  that  it  is  an  articu- 
late is  the  development  of  a  series  of  indentations  divid- 


DEVELOPMENT 


ing  the  body  into  successive  rings,  or  joints.  Next,  we 
observe  that  the  back  lies  near  the  center  of  the  egg,  the 
ventral  side  looking  outward,  i.e.,  the  embryo  is  doubled 
upon  itself  backward.  And,  finally,  the  appearance  of 
three  pairs  of  legs  proves  that  it  will  be  an  insect,  rather 
than  a  worm,  crustacean,  or  spider. 

The  vertebrate  embryo  lies  with  its  stomach  toward 
the  yolk,  reversing  the  position  of  the  articulate  ;  but 
the  grand  characteristic  is  the  medullary  groove,  which 
does  not  exist  in  the  egg  of  any  invertebrate.  This 
feature  is  connected  with  another,  the  setting  apart  of 
two  distinct  regions  —  the  nervous  and  nutritive.  There 
are  three  modifications  of  vertebrate  development  :  that 
of  fishes  and  amphibians,  that  of  true  reptiles  and 
birds,  and  that  of  mammals.  The  amnion  and  allantois 
are  wanting  in  the  first  group  ;  while  the  placenta  (which 
is  the  allantois  vitally  connected  with  the  parent)  is  pe- 
culiar to  mammals.  In  mammals,  the  whole  yolk  is  seg- 
mented ;  in  birds,  segmentation  is  confined  to  the  small 
white  speck  (blastoderm]  seen  in  opening  the  shell. 

At  the  outset,  all  animals,  from  the  sponge  to  man, 
are  structurally  alike.  All,  moreover,  undergo  segmen- 
tation, and  most  have  one  form  or  other  of  the  gastrula 
stage.  But  while  vertebrates  and  invertebrates  can 
travel  together  on  the  same  road  up  to  this  point,  here 
they  diverge  —  never  to  meet  again.  For  every  grand 
group  early  shows  that  it  has  a  peculiar  type  of  con- 
struction. Every  egg  is  from  the  first  impressed  with 
the  power  of  developing  in  one  direction  only,  and  never 
does  it  lose  its  fundamental  characters.  The  germ  of 
the  bee  is  divided  into  segments,  showing  that  it  belongs 
to  the  articulates  ;  the  germ  of  the  lion  has  the  medul- 
lary furrow  —  the  mark  of  the  coming  vertebrate.  The 
blastodermic  layer  of  the  vertebrate  egg  rolls  up  into 
two  tubes  —  one  to  hold  the  viscera,  the  other  to  con- 
DODGE'S  GEN.  ZOOL.  —  27 


4I8  COMPARATIVE  ZOOLOGY 

tain  the  nervous  cord :  while  that  of  the  invertebrate 
egg  forms  only  one  such  tubular  division.  The  features 
which  determine  the  branch  to  which  an  animal  belongs 
are  first  developed,  then  the  characters  revealing  its 
class. 

There  are  differences  also  in  grade  of  development  as 
well  as  type.  For  a  time  there  is  no  essential  difference 
between  a  fish  and  a  mammal;  they  have  similar  ner- 
vous, circulatory,  and  digestive  systems.  There  are 
many  such  cases,  in  which  the  embryo  of  an  animal 
represents  the  permanent  adult  condition  of  some  lower 
form.  In  other  words,  the  higher  species,  in  the  course 
of  their  development,  offer  likenesses,  or  analogies,  to 
finished  lower  species.  The  human  germ  at  first  re- 
sembles that  of  all  other  metazoa  in  that  it  is  a  single 
cell.  In  the  course  of  its  development,  the  appearance 
of  a  medullary  furrow  excludes  it  at  once  from  all  inver- 
tebrates. It  afterward  has,  for  a  time,  structures  found 
as  permanent  organs  in  the  lower  classes  and  orders  of 
vertebrates.  For  a  time,  indeed,  the  human  embryo  so 
closely  resembles  that  of  the  lower  forms  as  to  be  indis- 
tinguishable from  them ;  but  certain  structures  belong- 
ing to  those  forms  are  kept  long  after  the  embryo  is 
clearly  human.  For  instance,  the  embryos  of  birds  and 
mammals  at  an  early  stage  have  gill  slits,  like  fishes. 
Not  all  the  members  of  a  group  reach  the  same  degree 
of  perfection,  some  remaining  in  what  corresponds  to 
the  immature  stages  of  the  higher  animals.  Such  may 
be  called  permanently  embryonic  forms. 

Sometimes  an  embryo  develops  an  organ  in  a  rudimen- 
tary condition,  which  is  lost  or  useless  in  the  adult. 
Thus,  the  Greenland  whale,  when  grown  up,  has  not  a 
tooth  in  its  head,  while  in  the  embryo  life  it  has  teeth  in 
both  jaws ;  unborn  calves  have  canines  and  upper  in- 
cisors ;  and  the  female  dugong  has  tusks  which  never 


DEVELOPMENT 


cut  the  gum.     The  "  splint  bones  "  in  the  horse's  foot 
are  undeveloped  metatarsals. 

Animals  differ  widely  in  the  degree  of  development 
reached  at  ovulation  and  at  birth.  The  eggs  of  frogs 
are  laid  when  they  can  hardly  be  said  to  have  become 
fully  formed  as  eggs,  since  they  undergo  still  further 
change  in  the  water.  The  eggs  of  birds  are  laid  when 
segmentation  is  far  advanced,  while  the  eggs  of  mam- 
mals are  retained  by  the  parent  till  after  the  egg  stage 
is  passed.169  Ruminants  and  terrestrial  birds  are  born 
with  the  power  of  sight  and  locomotion.  Most  carni- 
vores, rodents,  and  perching  birds  come  into  the  world 
blind  and  helpless  ;  while  the  human  infant  is  depen- 
dent for  a  much  longer  time. 

I  .    Metamorphosis 

Few  animals  come  forth  from  the  egg  in  perfect  con- 
dition. The  vast  majority  pass  through  a  great  variety 
of  forms  before  reaching  maturity.  These  metamor- 
phoses (which  are  merely  periods  of  growth)  are  not 
peculiar  to  insects,  though  more  apparent  in  them. 
Man  himself  is  developed  on  the  same  general  principles 
as  the  butterfly,  but  the  transformations  take  place 
gradually.  The  coral,  when  hatched,  has  six  pairs  of 
partitions  ;  afterward,  the  spaces  are  divided  by  six 
more  pairs;  then  twelve  intermediate  pairs  are  intro- 
duced ;  next,  twenty-four,  and  so  on.  The  embryonic 
starfish  has  a  long  body,  with  six  arms  on  a  side,  in  one 
end  of  which  the  young  starfish  is  developed.  Soon  the 
twelve-armed  body  is  absorbed,  and  the  young  animal  is 
perfectly  formed.  Worms  are  continually  growing  by 
the  addition  of  new  segments.  Nearly  all  insects 
undergo  complete  metamorphosis,  i.e.,  exhibit  four  dis- 
tinct stages  of  existence  —  egg,  larva,  pupa,  and  imago. 


420 


COMPARATIVE   ZOOLOGY 


The  wormlike  larva  17°  may  be  called  a  locomotive  egg. 

It  has  little  resemblance  to  the  parent  in  structure  or 

habits,  eating  and 
growing  rapidly. 
Then  it  enters  the 
pupa  state,  wrapping 
itself  in  a  cocoon,  or 
case,  and  remaining 
apparently  dead  till 
new  organs  are  devel- 
oped, when  it  escapes 
a  perfect  winged  in- 
sect, or  imago.171 
Wings  never  exist  ex- 
ternally in  the  larva ; 
and  some  insects 

i  10.368. — Milkweed  butterfly:  A,  head  of  young  larva; 
B,  larva;  C,  pupa;  D,  imago;  E,  egg  (magnified),     which        Undergo        no 

apparent   metamorphosis,  as   lice,   are  wingless.      The 


3 


FIG.  369.  —  Metamorphosis  of  the  Mosquito  (Culex  pipiens}'.  A,  boat  of  eggs  ;  B,  some 
of  the  eggs  highly  magnified  ;  d,  with  lid  open  for  the  escape  of  the  larva,  C  ; 
D,  pupa;  E,  larva  magnified,  showing  respiratory  tube,  e,  anal  fins,  f,  antennae,  £•; 
F,  imago;  a,  antennae;  b,  beak. 


DEVELOPMENT 


421 


grasshopper  develops  from  the  young  larva  to  the 
winged  adult  without  changing  its  mode  of  life.  In 
the  development  of  the  common  crab,  so  different  is 
the  outward  form  of  the  newly  hatched  embryo  from 
that  of  the  adult,  that  the  former  has  been  described  as 
a  distinct  species. 

The    most    remarkable   example   of    metamorphosis 
among  vertebrates  is  furnished  by  the  amphibians.     A 


FIG.  370.  —  Metamorphosis  of  the  Newt. 

tadpole  —  the  larva  of  a  frog- — has  a  tail,  but  no  legs; 
gills,  instead  of  lungs ;  a  heart  precisely  like  that  of  the 
fish  ;  a  horny  beak  for  eating  vegetable  food,  and  a 
spiral  intestine  to  digest  it.  As  it  matures,  the  hinder 
legs  show  themselves,  then  the  front  pair;  the  beak 
falls  off ;  the  tail  and  gills  waste  away ;  lungs  are 
formed ;  the  digestive  apparatus  is  changed  to  suit  an 
animal  diet;  the  heart  is  altered  to  the  reptilian  type 
by  the  addition  of  another  auricle ;  in  fact,  skin,  mus- 
cles, nerves,  bones,  and  blood  vessels  vanish,  being  ab- 


422 


COMPARATIVE   ZOOLOGY 


sorbed  atom  by  atom,  and  a  new  set  is  substituted. 
Molting,  or  the  periodical  renewal  of  epidermal  parts, 
as  the  shell  of  the  lobster,  the  skin  of  the  toad,  the 
scales  of  snakes,  the  feathers  of  birds,  and  the  hair  of 
mammals,  may  be  termed  a  metamorphosis.  The  change 
from  milk  teeth  to  a  permanent  set  is  another  example. 
An  animal  rises  in  organization  as  development  ad- 
vances. Thus,  a  caterpillar's  life  has  nothing  nobler 
about  it  than  the  ability  to  eat,  while  the  butterfly  ex- 
pends the  power  garnered  up  by  the  larva  in  a  gay  and 
busy  life.  But  there  are  seeming  reversals  of  this  law. 
Some  mature  animals  appear  lower  in  the  scale  than 
their  young.  The  larval  cirripede  has  a  pair  of  mag- 
nificent compound  eyes  and  complex  antennae  ;  when 
adult,  the  antennae  are  gone,  and  the  eyes  are  reduced 
to  a  single,  simple,  minute  eye  spot.  The  germs  of  the 
sedentary  sponge  and  oyster  are  free  and  active.  The 
adult  animal,  however,  is  superior  in  alone  possessing 
the  power  of  reproduction.  Such  a  change  from  an 
active  to  a  fixed  condition  is  known  as  retrograde  meta- 
morphosis. 

There  are  certain  larval  forms  so  characteristic  of 
the  great  groups  of  the  animal  kingdom  as  to  demand 
notice.  Most  worms  leave  the  egg  as  a  larva,  called 
the  trochosphere  (Fig.  371),  an 
oval  larva,  having  mouth  and 
anus,  and  a  circle  of  cilia  an- 
terior to  the  mouth.  This 

f  FiG.372.-Veliger 

sphere  of  Worm,      larval      Stage      IS      COmmon      tO       Of  Snail,  magni- 
magnified  :       nt,  •    i        i  i«  fied  •    i>    velum  • 

mouth;  <z,anus;    worms  with  the  most  diverse  ' 


c,  circle  of  cilia.        ^^    forms     and     habjts>         ft 

is  also  found  in  many  of  the  mollusks.  The  mollusks 
usually  pass  through  a  later  stage  called  the  veliger 
(Fig.  372),  in  which  a  circle  of  cil-ia  homologous  to 
that  of  the  trochosphere  is  borne  by  a  lobed  expansion 


DEVELOPMENT 


423 


on  the  head,  called  the  velum,  or  sail.  The  Crustacea, 
which  exhibit  so  great  a  range  of  form  in  the  adult 
state,  all  pass  through  a  stage  in  which  they  are  sub- 
stantially alike.  Forms  as  different  in  appearance  as 
barnacles,  entomostracans,  and  prawns  hatch  out  as 
Nauplii,  little  oval  animals,  with  a  straight  intestine, 
three  pairs  of  legs,  and  a  simple  eye  (Fig.  373).  See 


FlG.  373.  —  Nauplius  of  Entomostracan  (Canthocamptus).  See  Fig.  57.  A,  first 
antenna;  An,  second  antenna;  a,  anus;  L,  labrum;  O,  ocellus;  S,  stomach.  (From 
Brooks,  after  Hoek.)  Magnified. 

Figures  56,  57,  58,  59.  Figure  59  represents  the  lobster, 
which  does  not  hatch  as  a  Nauplius,  but  is  not  very 
unlike  the  prawn.  These  larval  forms  are  of  great 
interest,  because  they  disclose  the  relationships  of  the 
adult  forms,  as  the  gastrula  stage  hints  at  the  common 
relationships  of  all  animals  above  Protozoa. 


424  COMPARATIVE   ZOOLOGY 

«  2.    Alternate  Generation 

Sometimes  a  metamorphosis  extending  over  several 
generations  is  required  to  evolve  the  perfect  animal; 
"  in  other  words,  the  parent  may  find  no  resemblance  to 
himself  in  any  of  his  progeny,  until  he  comes  down  to 
the  great-grandson."  Thus,  the  jellyfish,  or  medusa, 
lays  eggs  which  are  hatched  into  larvae  resembling 
Infusoria  —  little  transparent  oval  bodies  covered  with 
cilia,  by  which  they  swim  about  for  a  time  till  they  find 
a  resting  place.  One  of  them,  for  example,  becoming 
fixed,  develops  rapidly ;  it  elongates  and  spreads  at  the 
upper  end ;  a  mouth  is  formed,  opening  into  a  digestive 
cavity ;  and  tentacles  multiply  till  the  mouth  is  sur- 
rounded by  them.  At  this  stage  it  resembles  a  hydra. 
Then  slight  wrinkles  appear  along  the  body,  which 
grow  deeper  and  deeper,  till  the  animal  looks  like  "  a 
pine  cone  surmounted  by  a  tuft  of  tentacles";  and  then 
like  a  pile  of  saucers  (about  a  dozen  in  number)  with 
scalloped  edges.  Next,  the  pile  breaks  up  into  separate 
segments,  which  are,  in  fact,  so  many  distinct  animals ; 
and  each  turning  over  as  it  is  set  free,  so  as  to  bring  the 
mouth  below,  develops  into  an  adult  medusa,  becoming 
more  and  more  convex,  and  furnished  with  tentacles, 
circular  canals,  and  other  organs  exactly  like  those  of 
the  progenitor  which  laid  the  original  egg  (Figs.  20, 

374)- 

Here  we  see  a  medusa  producing  eggs  which  develop 
into  stationary  forms  resembling  hydras.  The  hydras 
then  produce  not  only  medusae  by  budding  in  the  man- 
ner described,  but  also  other  hydras  like  themselves  by 
budding.  All  these  intermediate  forms  are  transient 
states  of  the  jellyfish,  but  the  metamorphoses  can  not  be 
said  to  occur  in  the  same  individual.  While  a  cater- 
pillar becomes  a  butterfly,  this  hydralike  individual  pro- 


DEVELOPMENT 


425 


duces  a  number  of  medusae.  Alternate  generation  is, 
then,  an  alternation  of  asexual  and  sexual  methods  of 
reproduction,  one  or  more  generations  produced  from 
buds  being  followed  by  a  single  generation  produced 


FIG.  374. — Alternate  generation :  a,  b,  c,  ova  of  an  Acaleph  (Ckrysaora);  d,e,f,  Hydras; 
g,  ht  Hydras  with  constrictions;  z,  Hydra  undergoing  fission;  k,  one  of  the  separated 
segments,  a  free  medusa. 

from  eggs.  Often,  as  in  the  fresh-water  hydra,  the  two 
kinds  of  generations  are  alike  in  appearance.  The 
process  is  as  widespread  as  asexual  reproduction,  being 
found  mostly  in  sponges,  ccelenterates,  and  worms.  It 
is  also  found  in  certain  Crustacea  and  insects.  The 
name  is  sometimes  limited  to  cases  where  the  two  kinds 
of  generations  differ  in  form. 


3.    Growth  and  Repair 

Growth  is  increase  of  bulk,  as  Development  is  increase 
of  structure.  It  occurs  whenever  the  process  of  repair 
exceeds  that  of  waste,  or  when  new  material  is  added 
faster  than  the  tissues  are  destroyed.  There  is  a  specific 
limit  of  growth  for  all  animals,  although  many  of  the 
low  cold-blooded  forms,  as  the  trout  and  anaconda,  seem 
to  grow  as  long  as  they  live.  After  the  body  has  at- 
tained its  maturity,  i.e.,  has  fully  developed,  the  tissues 
cease  to  grow ;  and  nutrition  is  concerned  solely  in  sup- 


426  COMPARATIVE  ZOOLOGY 

plying  the  constant  waste,  in  order  to  preserve  the  size 
and  shape  of  the  organs.  A  child  eats  to  grow  and 
repair;  the  adult  eats  only  to  repair.172  Birds  develop 
rapidly,  and  so  spend  most  of  their  life  full-fledged; 
while  insects  generally,  fishes,  amphibians,  reptiles,  and 
mammals  mature  at  a  comparatively  greater  age.  The 
perfect  insect  rarely  changes  its  size,  and  takes  but 
little  food ;  eating  and  growing  are  almost  confined  to 
larval  life.  The  crust  of  the  sea  urchin,  which  is  never 
shed,  grows  by  the  addition  of  matter  to  the  margins  of 
the  plates.  The  shell  of  the  oyster  is  enlarged  by  the 
deposition  of  new  laminae,  each  extending  beyond  the 
other.  At  every  enlargement,  the  interior  is  lined  with 
a  new  nacreous  layer ;  so  that  the  number  of  such  layers 
in  the  oldest  part  of  the  shell  indicates  the  number  of 
enlargements.  When  the  shell  has  reached  its  full  size, 
new  layers  are  added  to  the  inner  surface  only,  which 
increases  the  thickness.  It  is  the  margin  of  the  mantle 
which  provides  for  the  increase  in  length  and  breadth, 
while  the  thickness  is  derived  from  the  whole  surface. 
The  edges  of  the  concentric  laminae  are  the  "  lines  of 
growth."  The  oyster  is  full-grown  in  about  five  years. 
The  bones  of  fishes  and  reptiles  are  continually  grow- 
ing ;  the  long  bones  of  higher  animals  increase  in  length 
so  long  as  the  ends  (epiphyses)  are  separate  from  the 
shaft.  The  limbs  of  man,  after  birth,  grow  more  rapidly 
than  the  trunk. 

The  power  of  regenerating  lost  parts  is  greatest  where 
the  organization  is  lowest,  and  while  the  animal  is  in  the 
young  or  larval  state.  It  is  really  a  process  of  budding. 
The  upper  part  of  the  hydra,  if  separated,  will  repro- 
duce the  rest  of  the  body;  if  the  lower  part  is  cut  off, 
it  will  add  the  rest.  Certain  worms  may  be  cut  into 
several  pieces,  and  each  part  will  regain  what  is  needed 
to  complete  the  mangled  organism.  The  starfish  can 


DEVELOPMENT  427 

reproduce  its  arms ;  the  holothurian,  its  stomach ;  the 
snail,  its  tentacles ;  the  lobster,  its  claws ;  the  spider,  its 
legs  ;  the  fish,  its  fins ;  and  the  lizard,  its  tail.  Nature 
makes  no  mistake  by  putting  on  a  leg  where  a  tail  be- 
longs, or  joining  an  immature  limb  to  an  adult  animal.173 
In  birds  and  mammals,  the  power  is  limited  to  the  repro- 
duction of  certain  tissues,  as  shown  in  the  healing  of 
wounds.  Very  rarely  an  entire  human  bone,  removed 
by  disease  or  surgery,  has  been  restored.  The  nails  and 
hair  continue  to  grow  in  extreme  old  age. 

4.   Likeness  and  Variation 

It  is  a  great  law  of  reproduction  that  all  animals  tend 
to  resemble  their  parents.  A  member  of  one  class  never 
produces  a  member  of  another  class.  The  likeness  is 
very  accurate  as  to  general  structure  and  form.  But  it 
does  not  descend  to  every  individual  feature  and  trait. 
In  other  words,  the  tendency  to  repetition  is  qualified 
by  a  tendency  to  variation.  Like  produces  like,  but  not 
exactly.  The  similarity  never  amounts  to  identity.  So 
that  we  have  two  opposing  tendencies  —  the  hereditary 
tendency  to  copy  the  original  stock,  and  a  distinct  ten- 
dency to  deviate  from  it. 

This  is  one  of  the  most  universal  facts  in  nature. 
Every  development  ends  in  diversity.  All  know  that 
no  two  individuals  of  a  family,  human  or  brute,  are 
absolutely  alike.  There  are  always  individual  differ- 
ences by  which  they  can  be  distinguished.  Evidently  a 
parent  does  not  project  precisely  the  same  line  of  influ- 
ences upon  each  of  its  offspring. 

This  variability  makes  possible  an  indefinite  modifica- 
tion of  the  forms  of  life.  For  the  variation  extends  to 
the  whole  being,  even  to  every  organ  and  mental  char- 
acteristic as  well  as  to  form  and  color.  It  is  very  slight 


428  COMPARATIVE   ZOOLOGY 

from  generation  to  generation ;  but  it  can  be  accumu- 
lated by  choosing  from  a  large  number  of  individuals 
those  which  possess  any  given  variation  in  a  marked 
degree,  and  breeding  from  these.  Nature  does  this  by 
the  very  gradual  process  of  "  natural  selection  "  ;  man 
hastens  it,  so  to  -speak,  by  selecting  extreme  varieties. 
Hence  we  have  in  our  day  remarkable  specimens  of 
poultry,  cattle,  and  dogs,  differing  widely  from  the  wild 
races. 

Sometimes  we  notice  that  children  resemble,  not  their 
parents,  but  their  grandparents  or  remoter  ancestors. 
This  tendency  to  revert  to  an  ancestral  type  is  called 
atavism.  Occasionally  stripes  appear  on  the  legs  and 
shoulders  of  the  horse,  in  imitation  of  the  aboriginal 
horse,  which  was  striped  like  the  zebra.  Sheep  have  a 
tendency  to  revert  to  dark  colors. 

The  laws  governing  inheritance  are  unknown.  No 
one  can  say  why  one  peculiarity  is  transmitted  from 
father  to  son,  and  not  another ;  or  why  it  appears  in  one 
member  of  the,family,  and  not  in  all.  Among  the  many 
causes  which  tend  to  modify  animals  after  birth  are  the 
quality  and  quantity  of  food,  amount  of  temperature 
and  light,  pressure  of  the  atmosphere,  nature  of  the  soil 
or  water,  habits  of  fellow  animals,  etc. 

Occasionally  animals  occur,  widely  different  in  struc- 
ture, having  a  very  close  external  resemblance.  Barna- 
cles were  long  mistaken  for  mollusks,  polyzoans  for 
polyps,  and  lamprey  eels  for  worms.  Such  forms  are 
termed  homomorpJiic. 

Members  of  one  group  often  put  on  the  outward  ap- 
pearance of  allied  species  in  the  same  locality ;  this  is 
called  mimicry.  "  They  appear  like  actors  or  masquer- 
aders  dressed  up  and  painted  for  amusement,  or  like 
swindlers  endeavoring  to  pass  themselves  off  for  well- 
known  and  respectable  members  of  society."  Thus, 


DEVELOPMENT  429 

certain  butterflies  on  the  Amazon  have  such  a  strong 
odor  that  the  birds  let  them  alone ;  and  butterflies  of  an- 
other family  in  the  same  region  have  assumed  for  pro- 
tection the  same  form  and  color  of  wing,  but  lack  the 
odor.  So  we  have  beelike  moths,  beetlelike  crickets, 
wasplike  flies,  and  antlike  spiders  ;  harmless  and  venom- 
ous snakes  copying  each  other,  and  orioles  departing 
from  their  usual  gay  coloring  to  imitate  the  plumage, 
flight,  and  voice  of  quite  another  kind  of  birds.  The 
species  which  are  imitated  are  much  more  abundant  than 
those  which  mimic  them.  There  is  also  a  general  har- 
mony between  the  colors  of  an  animal  and  those  of  its 
habitation  {protective  resemblance).  We  have  the  white 
polar  bear,  the  sand-colored  camel,  and  the  dusky  twilight 
moths.  There  are  birds  and  reptiles  so  tinted  and  mottled 
as  exactly  to  match  the  rock,  or  ground,  or  bark  of  a 
tree  they  frequent ;  and  there  are  insects  rightly  named 
"walking  sticks"  and  "walking  leaves."  These  coin- 
cidences are  often  beneficial  to  the  imitating  species. 
Generally,  they  wear  the  livery  of  those  they  live  on, 
or  resemble  the  forms  more  favored  than  themselves. 

Again,  some  animals  which  have  a  nauseous  taste  or 
odor,  as  certain  caterpillars,  insects,  salamanders,  etc., 
advertise  the  fact  by  being  brilliantly  colored  and  spotted 
(warning coloration),  and  are  thus  protected  against  other 
animals  which  would  prey  upon  them. 

5.    Homo  logy,  Analogy,  and  Correlation 

The  tendency  to  repetition  in  the  development  of 
animals  leads  to  some  remarkable  affinities.  Parts  or 
organs,  having  a  like  origin  and  development,  and 
therefore  the  same  essential  structure,  whatever  their 
form  or  function,  are  said  to  be  homologous  ;  while  parts 
or  organs  corresponding  in  use  are  called  analogous. 


430 


COMPARATIVE   ZOOLOGY 


By  serial  homology  is  meant  the  homology  existing  be- 
tween successive  parts  of  one  animal. 

The  following  are  examples  of  homology :  the  arms 
of  man,  the  fore  legs  of  a  horse,  the  paddles  of  a  whale, 
the  wings  of  a  bird,  the  front  flippers  of  a  turtle,  and  the 
pectoral  fins  of  a  fish ;  the  proboscis  of  a  moth,  and  the 
jaws  of  a  beetle;  the  shell  of  a  snail,  and  both  valves 
of  a  clam.  The  wings  of  the  bird,  flying  squirrel,  and 
bat  are  hardly  homologous,  since  the  wing  of  the  first  is 
developed  from  the  fore  limb  only ;  that  of  the  squirrel 
is  an  extension  of  the  skin  between  the  fore  and  hind 
limbs ;  while  in  the  bat  the  skin  stretches  between  the 
fingers,  and  then  down  the  side  to  the  tail.  Examples 
of  serial  homology :  the  arms  and  legs  of  man ;  the 
upper  and  lower  set  of  teeth ;  the  parts  of  the  vertebral 
column,  however  modified;  the  scapular  and  pelvic 
arches;  the  humerus  and  femur;  carpus  and  tarsus; 
the  right  and  left  sides  of  most  animals ;  the  dorsal  and 
anal  fins  of  fishes.  The  legs  of  a  lobster  and  lizard, 
the  wings  of  a  butterfly  and  bird,  the  gills  of  a  fish  and 
the  lungs  of  other  vertebrates,  are  analogous.  The  air 
bladder  of  a  fish  is  homologous  with  a  lung,  and  analo- 
gous to  the  air  chambers  of  the  nautilus. 

In  the  midst  of  the  great  variety  of  form  and  structure 
in  the  animal  world,  a  certain  harmony  reigns.  Not 
only  are  different  species  so  related  as  to  suggest  a 
descent  from  the  same  ancestor,  but  the  parts  of  any 
one  organism  are  so  closely  connected  and  mutually 
dependent  that  the  character  of  one  must  receive  its 
stamp  from  the  character  of  all  the  rest.  Thus,  from  a 
single  tooth  it  may  be  inferred  that  the  animal  had  a 
skeleton  and  spinal  cord,  and  that  it  was  a  carnivorous, 
hot-blooded  mammal.  Certain  structures  always  coexist. 
Animals  with  two  occipital  condyles,  and  non-nucleated 
blood  corpuscles,  suckle  their  young,  i.e.,  they  are  mam- 


DEVELOPMENT 


431 


mals.  All  ruminant  hoofed  beasts  have  horns  and 
cloven  feet.  If  the  hoofs  are  even,  the  horns  are  even, 
as  in  the  ox ;  if  odd,  as  in  the  rhinoceros,  the  horns  are 


FIG.  377. 


FIG.  378, 


HOMOLOGIES    OF   LIMBS 


FIG.  375.  —  Arm  and  Leg  of  Man,  as  they  are  when  he  gets  down  on  all  fours. 
FIG.  376.  —  Fore  and  Hind  Legs  of  Tapir.  FIG.  377.  —  Fore  Leg  of  Seal  and  Hind 
Leg  of  Alligator.  FIG.  378.  — Wing  of  the  Bat.  S,  scapula  ;  I,  ilium,  or  rim-bone 
of  pelvis  ;  H,  humerus  ;  F,  femur  ;  O,  olecranon,  or  tip  of  the  elbow  ;  P,  patella  ; 
U,  ulna  ;  T,  tibia  ;  R,  radius ;  Fi,  Fibula;  Po,  pollex,  or  thumb  ;  Ha,  hallux,  or  great 
toe.  Compare  the  fore,  and  hind  limbs  of  the  same  animal,  and  the  fore  or  hind  limbs 
of  different  animals.  Note  the  directions  of  the  homologous  segments. 


432  COMPARATIVE   ZOOLOGY 

odd,  i.e.,  single,  or  two  placed  one  behind-  the  other. 
Recent  creatures  with  feathers  always  have  beaks. 
Pigeons  with  short  beaks  have  small  feet;  and  those 
with  long  beaks,  large  feet.  The  long  limbs  of  the 
hound  are  associated  with  a  long  head.  A  white  spot 
in  the  forehead  of  a  horse  generally  goes  with  white 
feet.  Hairless  dogs  are  deficient  in  teeth.  Long  wings 
usually  accompany  long  tail  feathers.  White  cats  with 
blue  eyes  are  usually  deaf.  A  sheep  with  numerous 
horns  is  likely  to  have  long,  coarse  wool.  Homologous 
parts  tend  to  vary  in  the  same  manner ;  if  one  is  diseased, 
another  is  more  likely  to  sympathize  with  it  than  one 
not  homologous.  This  association  of  parts  is  called 
correlation  of  growth. 

6.    Individuality 

It  seems  at  first  sight  very  easy  to  define  an  individ- 
ual animal.  A  single  fish,  or  cow,  or  snail,  or  lobster  is 
plainly  an  individual ;  and  the  half  of  one  such  animal 
is  plainly  not  one.  But  when  we  consider  animals  in 
colonies,  like  corals,  it  is  not  so  easy  to  say  whether  the 
individual  is  the  colony  or  the  single  polyp.  Is  the  tree 
the  individual,  or  the  bud  ?  If  we  say  the  former  —  the 
colony  —  what  shall  we  say  to  the  free  buds  of  a  hydroid 
colony,  living  independent  lives,  and  scattered  over 
square  miles  of  ocean  ?  Are  they  parts  of  one  individ- 
ual ?  If  we  choose  the  latter  as  our  standard,  we  are  in 
equal  difficulty ;  for  we  must  then  call  an  individual  the 
bud  of  the  Portuguese  man-of-war,  reduced  to  a  mere 
bladder  or  feeler,  and  incapable  of  leading  an  indepen- 
dent life.  We  thus  find  it  necessary  to  distinguish  at 
least  two  kinds  of  individuals — -physiological  individ- 
uals,  applying  that  name  to  any  animal  form  capable 
of  leading  an  independent  life ;  and  morphological  indi- 
viduals, one  of  which  is  the  total  product  of  an  egg. 


DEVELOPMENT 


433 


Such  an  individual  may  be  a  single  physiological  indi- 
vidual, as  the  fish ;  or  many  united,  as  the  coral  stock ; 
or  many  separate  physiological  individuals,  as  in  the 
hydroids  or  plant  lice.  The  single  members  of  such  a 
compound  morphological  individual  are  called  zooids,  or 
persona,  and  are  found  wherever  asexual  reproduction 
takes  place. 

7.    Relations  of  Number,  Size,  Form,  and  Rank 

The  animal  kingdom  has  been  likened  to  a  pyramid, 
the  species  diminishing  in  number  as  they  ascend  in  the 
scale  of  complexity.  This  is  not  strictly  true.  The 
number  of  living  species  known  is  at  least  300,000,  of 
which  more  than  nine  tenths  are  invertebrates.  A  late 
enumeration  gives  the  following  figures  for  the  number 
of  described  species  :  — 


Protozoa 4,000 

Coslenterata 3>5°° 

Vermes 5»5^o 

Arthropoda 250,000 


Echinodermata   .  .  .     2,300 

Mollusca     21,000 

Vertebrata 25,200 


These  figures  are  lower  than  those  usually  given.  Of 
vertebrates,  fishes  are  most  abundant ;  then  follow  birds, 
mammals,  reptiles,  and  amphibians.  There  are  usually 
said  to  be  about  200,000  species  of  insects  and  it  is  esti- 
mated that  there  are  about  500,000  living  species  in  the 
animal  kingdom  ;  about  40,000  extinct  species  have  been 
described. 

The  largest  species  usually  belong  to  the  higher 
classes.  The  aquatic  members  of  a  group  are  generally 
larger  than  the  terrestrial,  the  marine  than  the  fresh- 
water, and  the  land  than  the  aerial.  The  extremes  of 
size  are  an  Infusorium,  ^Jo^  of  an  inch  in  diameter, 
and  the  whale,  eighty-five  feet  long,  respectively  the 
smallest  and  the  largest  animal  ever  measured.  The 
DODGE'S  GEN.  ZOOL.  —  28 


434 


COMPARATIVE   ZOOLOGY 


female  is  sometimes  larger  than  the  male,  as  of  the  nau- 
tilus, spider,  and  eagle.  The  higher  the  class,  the  more 
uniform  the  size.  Of  all  groups  of  animals,  insects  and 
birds  are  the  most  constant  in  their  dimensions. 

Every  organism  has  its  own  special  law  of  growth  :  a 
fish  and  an  oyster,  though  born  in  the  same  locality,  de- 
velop into  very  different  forms.  Yet  a  symmetry  of 
plan  underlies  the  structure  of  all  animals.  In  the 
embryo,  this  syrrimetry  of  the  two  ends,  as  well  as  the 
two  sides,  is  nearly  perfect ;  but  it  is  subsequently  inter- 
fered with  to  adapt  the  animal  to  its  special  conditions 
of  life.  It  is  a  law  that  an  animal  grows  equally  in 
those  directions  in  which  the  incident  forces  are  equal. 
The  polyp,  rooted  to  the  rocks,  is  subjected  to  like  con- 
ditions on  all  sides,  and,  therefore,  it  has  no  right  and 
left,  or  fore  and  hind  parts.  The  lower  forms,  generally, 
are  more  or  less  geometrical  figures :  spheroidal,  as  the 
sea  urchin ;  radiate,  as  the  starfish ;  and  spiral,  as  many 
foraminifers.  The  higher  animals  are  subjected  to  a 
greater  variety  of  conditions.  Thus,  a  fish,  always  go- 
ing through  the  water  head  foremost,  must  show  con- 
siderable difference  between  the  head  and  the  hinder 
end ;  or  a  turtle,  moving  over  the  ground  with  the  same 
surface  always  down,  must  have  distinct  dorsal  and 
ventral  sides. 

Nevertheless,  there  is  a  striking  likeness  between  the 
two  halves  or  any  two  organs  situated  on  opposite  sides 
of  an  axis.  And,  first,  a  bilateral  symmetry  is  most  com- 
mon. It  is  best  exhibited  by  the  arthropods  and  verte- 
brates, but  nearly  all  animals  can  be  clearly  divided  into 
right  and  left  sides  —  in  other  words,  they  appear  to  be 
double.  A  vertical  plane  would  divide  into  two  equal 
parts  our  brain,  spinal  cord,  vertebral  column,  organs  of 
sight,  hearing,  and  smell ;  our  teeth,  jaws,  limbs,  lungs, 
etc.  In  fact,  the  two  halves  of  every  egg  are  identical. 


DEVELOPMENT 


435 


There  are  many  exceptions :  the  heart  and  liver  of  the 
higher  vertebrates  are  eccentric ;  the  nervous  system  of 
mollusks  is  scattered;  the  hemispheres  of  the  human 
brain  are  sometimes  unequal ;  the  corresponding  bones 
in  the  right  and  left  arms  are  not  precisely  the  same 
length  and  weight ;  the  narwhal  has  an  immense  tusk  on 
the  left  side,  with  none  to  speak  of  on  the  other ;  the 
rattlesnake  has  but  one  lung,  the  second  remaining  in  a 
rudimentary  condition ;  both  eyes  of  the  adult  flounder 
and  halibut  are  on  the  same  side ;  the  claws  of  the  lob- 
ster differ;  and  the  valves  of  an  oyster  are  unequal. 
But  all  these  animals  and  their  organs  are  perfectly  sym- 
metrical in  the  embryo  state. 

Again,  animals  exhibit  a  certain  correspondence  be- 
tween the  fore  and  hind  parts.17*  Thus,  the  two  ends 
of  the  centipede  repeat  each  other.  Indeed,  in  some 
worms,  the  eyes  are  developed  in  the  last  segment  as 
well  as  the  first.  In  the  embryo  of  quadrupeds,  the 
four  limbs  are  closely  alike.  But  in  the  adult,  the  fore 
and  hind  limbs  differ  more  than  the  right  and  left  limbs, 
because  the  functions  are  more  dissimilar.  An  extreme 
want  of  symmetry  is  seen  in  birds,  which  combine  aerial 
and  land  locomotion. 

Every  animal  is  perfect  in  its  kind  and  in  its  place. 
Yet  we  recognize  a  gradation  of  life.  Some  animals  are 
manifestly  superior  to  some  others.  But  it  is  not  so 
easy  to  say  precisely  what  shall  guide  us  in  assorting 
living  forma  into  high  and  low.  Shall  we  make  structure 
the  criterion  of  rank?  Plainly  the  simple  jellyfish  is 
beneath  complicated  man.  The  intricate  and  finished 
build  of  the  horse  elevates  him  immeasurably  above  the 
stupid  snail.  The  repetition  of  similar  parts,  as  in  the 
worm,  is  a  sign  of  low  life.  So  also  a  prolonged  poste- 
rior is  a  mark  of  inferiority,  as  the  lobsters  are  lower 
than  the  crabs,  snakes  than  lizards,  monkeys  than  apes. 


436  COMPARATIVE   ZOOLOGY 

The  possession  of  a  head  distinct  from  the  region  behind 
it  is  a  sign  of  power.  And  in  proportion  as  the  fore 
limbs  are  used  independently  of  the  hind  limbs,  the 
animal  ascends  the  scale :  compare  the  whale,  horse, 
cat,  monkey,  and  man. 

But  shall  the  fish,  never  rising  above  the  "  monotony 
of  its  daily  swim,"  be  allowed  to  outrank  the  skillful  bee  ? 
Shall  the  brainless,  sightless,  almost  heartless  amphioxus, 
a  vertebrate,  be  allowed  to  stand  nearer  to  man  than  the 
ant  ?  What  is  the  possession  of  a  backbone  to  intelli- 
gence ?  No  good  reason  can  be  given  why  we  might  not 
be  just  as  intelligent  beings  if  we  carried,  like  the  insect, 
our  hearts  in  our  backs  and  our  spinal  cords  in  our 
breasts.  So  far  as  its  activity  is  concerned,  the  brain 
may  be  as  effective  if  spread  out  like  a  map  as  packed 
into  its  present  shape.  Even  animals  of  the  same  type, 
as  vertebrates,  can  not  be  ranked  according  to  complex- 
ity. For  while  mammals,  on  the  whole,  are  superior  to 
birds,  birds  to  reptiles,  and  reptiles  to  fishes,  they  are 
not  so  in  every  respect.  Man  himself  is  not  altogether 
at  the  head  of  creation.  We  carry  about  in  our  bodies 
embryonic  structures.  That  structural  affinity  and  vital 
dignity  are  not  always  parallel  may  be  seen  by  compar- 
ing an  Australian  aborigine  and  an  Englishman.175 

Function  is  the  test  of  worth.  Not  mere  work,  how- 
ever ;  for  we  must  consider  its  quality  and  scope.  An 
animal  may  be  said  to  be  more  perfect  in  proportion  as 
its  relations  to  the  external  world  are  more  varied,  pre- 
cise, and  fitting.  Complexity  of  organization,  variety, 
and  amount  of  power  are  secondary  to  the  degree  in 
which  the  whole  organism  is  adapted  to  the  circum- 
stances which  surround  it,  and  to  the  work  which  it  has 
to  do.  Ascent  in  the  animal  scale  is  not  a  passage  from 
animals  with  simple  organs  to  animals  with  complex 
organs,  but  from  simple  individuals  with  organs  of 


DEVELOPMENT  437 

complex  function  to  complex  individuals  with  organs 
of  simple  function :  the  addition  as  we  ascend  being  not 
function,  but  parts  to  discharge  those  functions  ;  and  the 
advantage  gained,  not  another  thing  done,  but  the  same 
thing  done  better.  Advance  in  rank  is  exhibited,  not 
by  the  possession  of  more  life  (for  some  animalcules  are 
ten  times  more  lively  than  the  busiest  man),  but  by  the 
setting  apart  of  more  organs  for  special  purposes. 
The  higher  the  animal,  the  greater  the  number  of  parts 
combining  to  perform  each  function.  The  power  is 
increased  by  this  division  of  labor.  The  most  impor- 
tant feature  in  this  specialization  is  the  tendency  to 
concentrate  the  nervous  energy  toward  the  head  (ceph- 
alizatiori).  It  increases  as  we  pass  from  lower  to  higher, 
animals. 

As  a  rule,  fixed  species  are  inferior  to  the  free,  water 
species  to  land  species,  fresh-water  animals  to  marine, 
arctic  forms  to  tropical,  and  the  herbivorous  to  the  car- 
nivorous. Precocity  is  a  sign  of  inferiority :  compare 
the  chicks  of  the  hen  and  the  robin,  a  colt  with  a  kitten, 
the  comparatively  well-developed  caterpillar  with  the 
footless  grub  of  the  bee.  Among  invertebrates,  the  male 
is  frequently  inferior,  not  only  in  size,  but  also  in  grade 
of  organization.  Animals  having  a  wide  range  as  to  cli- 
mate, altitude,  or  depth  are  commonly  inferior  to  those 
more  restricted  ;  man  is  a  notable  exception. 

There  is  some  relation  between  the  duration  of  life  and 
the  size,  structure,  and  rank  of  animals.  Vertebrates 
not  only  grow  to  a  greater  size,  but  also  live  longer  than 
invertebrates.  Whales  and  elephants  are  the  longest- 
lived  ;  and  falcons,  ravens,  parrots,  and  geese,  alligators 
and  turtles,  and  sharks  and  pikes  are  said  to  live  a  cen- 
tury. The  life  of  quadrupeds  generally  reaches  its  limit 
when  the  molar  teeth  are  worn  down  :  those  of  the  sheep 
last  about  1 5  years ;  of  the  ox,  20 ;  of  the  horse,  40 ;  of 


438 


COMPARATIVE   ZOOLOGY 


the  elephant,  100.  Many  inferior  species  die  as  soon  as 
they  have  laid  their  eggs,  just  as  herbs  perish  as  soon  as 
they  have  flowered. 

8.    The  Struggle  for  Life 

Every  species  of  animal  is  striving  to  increase  in  a 
geometrical  ratio.  But  each  lives,  if  at  all,  by  a  struggle 
at  some  period  of  its  life.  The  meekest  creatures  must 
fight,  or  die. 

"  There  is  no  exception  to  the  rule  that  every  organic 
being  naturally  increases  at  so  high  a  rate,  that,  if  not 
destroyed,  the  earth  would  soon  be  covered  by  the  prog- 
eny of  a  single  pair."  If  the  increase  of  the  human  race 
were  not  checked,  there  would  not  be  standing  room 
for  the  descendants  of  Adam  and  Eve.  A  pair  of  ele- 
phants, the  slowest  breeder  of  all  known  animals,  would 
become  the  progenitors,  in  seven  and  one-half  centuries, 
of  19,000,000  of  elephants,  if  death  did  not  interfere. 
Evidently  a  vast  number  of  young  animals  must  perish 
while  immature,  and  a  far  greater  host  of  eggs  fail  to 
mature.  A  single  cod,  laying  millions  of  eggs,  if  al- 
lowed to  have  its  own  way,  would  soon  pack  the  ocean. 

Yet,  so  nicely  balanced  are  the  forces  of  nature,  the 
average  number  of  each  kind  remains  about  the  same. 
The  total  extinction  of  any  one  species  is  exceedingly 
rare.  The  number  of  any  given  species  is  not  deter- 
mined by  the  number  of  eggs  produced,  but  by  its  sur- 
rounding conditions.176  Aquatic  birds  outnumber  the 
land  birds,  because  their  food  never  fails,  not  because 
they  are  more  prolific.  The  fulmer  petrel  lays  but  one 
egg,  yet  it  is  believed  to  be  the  most  numerous  bird  in 
the  world. 

The  main  checks  to  the  high  rate  of  increase  are: 
climate  (temperature  and  moisture),  acting  directly  or 
indirectly  by  reducing  food ;  and  other  animals,  either 


DEVELOPMENT  439 

rivals  requiring  the  same  food  and  locality,  or  enemies, 
for  the  vast  majority  of  animals  are  carnivorous.  Off- 
spring are  continually  varying  from  their  parents,  for 
better  or  worse.  If  feebly  adapted  to  the  conditions  of 
existence,  they  will  finally  go  to  the  wall.  But  those 
forms  having  the  slightest  advantage  over  others  in- 
habiting the  same  region,  being  hardier  or  stronger, 
more  agile  or  sagacious,  will  survive.  Should  this  ad- 
vantageous variation  become  hereditary  and  intensified, 
the  new  variety  will  gradually  extirpate  or  replace  other 
kinds.  This  is  what  Mr.  Darwin  means  by  Natural 
Selection,  and  Herbert  Spencer  by  the  Survival  of  the 
Fittest. 


CHAPTER   XXIV 

THE  DISTRIBUTION   OF  ANIMALS 

LIFE  is  everywhere.  In  the  air  above,  the  earth  be- 
neath, and  the  waters  under  the  earth,  we  are  surrounded 
with  life.  Nature  lives  :  every  death  is  only  a  new  birth, 
every  grave  a  cradle.  The  air  swarms  with  birds,  insects, 
and  invisible  animalcules.  The  waters  are  peopled  with 
innumerable  forms,  from  the  protozoan,  millions  of  which 
would  not  weigh  a  grain,  to  the  whale,  so  large  that  it 
seems  an  island  as  it  sleeps  upon  the  waves.  The  bed 
of  the  sea  is  alive  with  crabs,  mollusks,  polyps,  star- 
fishes, and  Foraminifera.  Life  everywhere  —  on  the 
earth,  in  the  earth,  crawling,  creeping,  burrowing,  bor- 
ing, leaping,  running. 

Nor  does  the  vast  procession  end  here.  The  earth 
we  tread  is  largely  formed  of  the  debris  of  life.  The 
quarry  of  limestone,  the  flints  which  struck  the  fire  of 
the  old  Revolutionary  muskets,  are  the  remains  of  count- 
less skeletons.  The  major  part  of  the  Alps,  the  Rocky 
Mountains,  and  the  chalk  cliffs  of  England  are  the  mon- 
umental relics  of  bygone  generations.  From  the  ruins 
of  this  living  architecture  we  build  our  Parthenons  and 
Pyramids,  our  St.  Peters  and  Louvres.  So  generation 
follows  generation.  But  we  have  not  yet  exhausted  the 
survey.  Life  cradles  within  life.  The  bodies  of  ani- 
mals are  little  worlds  having  their  own  fauna  and  flora. 
In  the  fluids  and  tissues,  in  the  eye,  liver,  stomach, 
brain,  and  muscles,  parasites  are  found ;  and  these 
parasites  often  have  their  parasites  living  on  them. 

44° 


THE   DISTRIBUTION    OF   ANIMALS  441 

Even  the  unicellular  forms,  Stylonychia,  for  example, 
have  been  found  to  be  infested  with  parasitic  protozoans. 

Thus  the  ocean  of  life  is  inexhaustible.  It  spreads 
in  every  direction,  into  time  past  and  present,  flowing 
everywhere,  eagerly  surging  into  every  nook  and  corner 
of  creation.  On  the  mountain  top,  in  the  abysses  of 
the  Atlantic,  in  the  deepest  crevice  of  the  earth's  crust, 
we  find  traces  of  animal  life.  Nature  is  prodigal  of 
space,  but  economical  in  filling.it.177 

Animals  are  distributed  over  the  globe  according  to 
definite  laws,  and  with  remarkable  regularity. 

Each  of  the  three  great  provinces,  Earth,  Air,  and 
Water,  as  also  every  continent,  contains  representatives 
of  all  the  classes ;  but  the  various  classes  are  unequally 
represented.  Every  great  climatal  region  contains  some 
species  not  found  elsewhere,  to  the  exclusion  of  some 
other  forms.  Every  grand  division  of  the  globe, 
whether  of  land  or  sea,  each  zone  of  climate  and  alti- 
tude, has  its  own  fauna.  In  traveling  over  the  earth 
and  settling  in  new  regions  man  has  been  accompanied 
by  many  animals  which  have  established  themselves 
and  thriven  in  the  land  of  their  adoption.  For  example, 
the  house,  or  "  English,"  sparrow  has  been  brought  to 
America,  and  the  sparrow  and  rabbit  to  New  Zealand. 
Hence,  it  is  necessary  to  distinguish  between  the  native 
or  indigenous  fauna,  and  the  introduced  fauna,  the  latter 
depending  upon  human  agency.  In  spite  of  the  many 
causes  tending  to  disperse  animals  beyond  their  natural 
limits,  each  country  preserves  its  peculiar  zoological 
physiognomy. 

The  space  occupied  by  the  different  groups  of  ani- 
mals is  often  inversely  as  the  size  of  the  individuals. 
Compare  the  coral  and  elephant. 

The  fauna  now  occupying  a  separate  area  is  closely 
allied  to  the  fauna  which  existed  in  former  geologic 


442  COMPARATIVE   ZOOLOGY 

times.  Thus,  Australia  has  always  been  the  home  of 
marsupials,  and  South  America  of  edentates. 

It  is  a  general  rule  that  groups  of  distinct  species  are 
circumscribed  within  definite,  and  often  narrow,  limits. 
Man  is  the  only  cosmopolitan ;  yet  even  he  comprises 
several  marked  races,  whose  distribution  corresponds 
with  the  great  zoological  regions.  The  natives  of  Aus- 
tralia are  as  grotesque  as  the  animals.  Certain  brutes 
likewise  have  a  great  range :  thus,  the  puma  ranges 
from  Canada  to  Patagonia;  the  muskrat,  from  the 
Arctic  Ocean  to  Florida;  the  ermine,  from  Bering 
Strait  to  the  Himalayas  ;  and  the  hippopotamus,  from 
the  Nile  and  Niger  to  the  Orange  River.178 

Frequently,  species  of  the  same  genus,  living  side 
by  side,  are  widely  different,  while  there  is  a  close  re- 
semblance between  forms  which  are  antipodes.  The 
mud  eel  of  South  Carolina  and  menobranchus  of  the 
Northern  States  have  their  relatives  in  Japan  and  Aus- 
tria. The  American  tapir  has  its  mate  in  Sumatra,  the 
llama  is  related  to  the  camel,  and  the  opossum  to  the 
kangaroo. 

The  chief  causes  modifying  distribution  are  tempera- 
ture, topography,  ocean  and  wind  currents,  humidity, 
and  light.  To  these  may  be  added  the  fact  that  ani- 
mals are  ever  intruding  on  each  other's  spheres  of  exist- 
ence. High  mountain  ranges,  wide  deserts,  and  cold 
currents  in  the  ocean  are  impassable  barriers  to  the 
migration  of  most  species.  Thus,  river  fish  on  opposite 
sides  of  the  Andes  differ  widely,  and  the  cold  Peruvian 
current  prevents  the  growth  of  coral  at  the  Galapagos 
Islands.  So  a  broad  river,  like  the  Amazon,  or  a  deep, 
narrow  channel  in  the  sea,  is  an  effectual  barrier  to 
some  tribes.  Thus,  Borneo  belongs  to  the  Indian  region, 
while  Celebes,  though  but  a  few  miles  distant,  is  Aus- 
tralian in  its  life.  The  faunae  of  North  America,  on 


THE   DISTRIBUTION    OF   ANIMALS  443 

the  east  coast,  west  coast,  and  the  open  plains  between, 
are  very  different. 

Animals  dwelling  at  high  elevations  resemble  those 
of  colder  latitudes.  The  same  species  of  insects  are 
found  on  Mount  Washington,  and  in  Labrador  and 
Greenland. 

The  range  does  not  depend  upon  the  powers,  of  loco- 
motion. The  oyster  extends  from  Halifax  to  Charles- 
ton, and  the  snapping  turtle  from  Canada  to  the 
equator;  while  many  quadrupeds  and  birds  have  nar- 
row habitats. 

The  distribution  of  any  group  is  qualified  by  the 
nature  of  the  food.  Carnivores  have  a  wider  range 
than  herbivores. 

Life  diminishes  as  we  depart  from  the  equator  north 
or  south,  and  likewise  as  we  descend  or  ascend  from 
the  level  of  the  sea. 

The  zones  of  geography  have  been  divided  by  zoolo- 
gists into  narrower  provinces.  Three  regions  in  the 
sea  are  recognized :  the  Pelagic  or  surface  region ;  the 
Littoral,  between  tide  marks  strictly  but  often  inter- 
preted to  conclude  depths  to  forty  fathoms  ;  and  the 
Abyssal,  extending  from  the  Littoral  to  the  greatest 
depths  of  the  ocean.  Every  marine  species  has  its  own 
limits  of  depth.  It  would  be  quite  as  difficult,  said 
Agassiz,  for  a  fish  or  a  mollusk  to  cross  from  the  coast 
of  Europe  to  the  coast  of  America  as  for  a  reindeer  to 
pass  from  the  arctic  to  the  antarctic  regions  across  the 
torrid  zone.  Marine  animals  congregate  mainly  along 
the  coasts  of  continents  and  on  soundings.  The  meet- 
ing place  of  two  maritime  currents  of  different  tem- 
peratures, as  on  the  Banks  of  Newfoundland,  favors 
the  development  of  a  great  diversity  of  fishes. 

Every  great  province  of  the  ocean  contains  some 
representatives  of  all  the  subkingdoms.  Deep-sea  life 


444  COMPARATIVE   ZOOLOGY 

is  diversified,  though  comparatively  sparse.  Examples 
of  all  the  five  invertebrate  divisions  were  found  in  the 
Bay  of  Biscay,  at  the  depth  of  2435  fathoms.179 

Distribution  in  the  sea  is  influenced  by  the  tempera- 
ture and  composition  of  the.  water  and  the  character  of 
the  bottom.  The  depth  acts  indirectly  by  modifying 
the  temperature.  Northern  animals  approach  nearer 
to  the  equator  in  the  sea  than  on  the  land,  on  account 
of  cold  currents.  The  heavy  aquatic  mammals,  as 
whales,  walruses,  seals,  and  porpoises,  are  mainly 
polar. 

The  land  consists  of  the  following  somewhat  distinct 
areas :  the  Neotropic,  comprising  South  America,  the 
West  Indies,  and  most  of  Mexico ;  the  Nearctic,  includ- 
ing the  rest  of  America;  the  Palearctic,  composed  of 
the  eastern  continent  north  of  the  Tropic  of  Cancer, 
and  the  Himalayas ;  the  Ethiopian,  or  Africa  south  of 
the  Tropic  of  Cancer ;  the  Oriental,  or  India,  the  south- 
ern part  of  China,  the  Malay  Peninsula,  and  the  islands 
as  far  east  as  Java,  Borneo,  and  the  Philippine  Islands ; 
and  the  Australian,  or  the  eastern  half  of  the  Malay 
Islands  and  Australia.  These  are  the  regions  of  Sclater 
and  Wallace.  Other  writers  unite  the  northern  parts  of 
both  hemispheres  into  one  region,  and  the  Oriental  with 
the  Ethiopian  regions. 

Life  in  the  polar  regions  is  characterized  by  great 
uniformity,  the  species  being  few  in  number,  though 
the  number  of  individuals  is  immense.  The  same  ani- 
mals inhabit  the  arctic  portions  of  the  three  continents ; 
while  the  antarctic  ends  of  the  continents,  Australia, 
Cape  of  Good  Hope,  and  Cape  Horn,  exhibit  strong 
contrasts.  Those  three  continental  peninsulas  are,  zoo- 
logically, separate  worlds.  In  fact,  the  whole  southern 
hemisphere  is  peculiar.  Its  fauna  is  antique.  Aus- 
tralia possesses  a  strange  mixture  of  the  old  and  new. 


THE   DISTRIBUTION   OF   ANIMALS  445 

South  America,  with  newer  mammals,  has  older  reptiles; 
while  Africa  has  a  rich  vertebrate  life,  with  a  striking 
uniformity  in  its  distribution.  Groups,  old  geologically 
and  now  nearly  extinct,  are  apt  to  have  a  peculiar  dis- 
tribution ;  as  the  Edentata  in  South  America,  Africa, 
and  India ;  the  marsupials  in  Australia  and  America ; 
the  Ratitae  in  South  America,  Africa,  Australia,  and 
New  Zealand. 

In  the  tropics,  diversity  is  the  law.  Life  is  more 
varied  and  crowded  than  elsewhere,  and  attains  its 
highest  development. 

The  New  World  fauna  is  old-fashioned,  and  inferior 
in  rank  and  size,  compared  with  that  of  the  eastern 
continents. 

As  a  rule,  the  more  isolated  a  region  the  greater  the 
variety.  Oceanic  islands  have  comparatively  few  species, 
but  a  large  proportion  of  endemic  or  peculiar  forms. 
Batrachians  are  absent,  and  there  are  no  indigenous 
terrestrial  mammals.  The  productions  are  related  to 
those  of  the  nearest  continent.  When  an  island,  as 
Britain,  is  separated  from  the  mainland  by  a  shallow 
channel,  the  mammalian  life  is  the  same  on  both  sides. 

Protozoans,  ccelenterates,  and  echinoderms  are  limited 
to  the  waters,  and  nearly  all  are  marine.  Sponges  are 
mostly  obtained  from  the  Grecian  Archipelago  and 
Bahamas,  but  species  not  commercially  valuable  abound 
in  all  seas.  Coral  reefs  abound  throughout  the  Indian 
Ocean  and  Polynesia,  east  coast  of  Africa,  Red  Sea, 
and  Persian  Gulf,  West  Indies,  and  around  Florida; 
and  corals  which  do  not  form  reefs  are  much  more 
widely  distributed,  being  found  as  far  north  as  Long 
Island  Sound  and  England.  Crinoids  have  been  found, 
usually  in  deep  sea,  in  very  widely  separated  parts  of 
the  world  —  off  the  coast  of  Norway,  Scotland,  and 
Portugal,  and  near  the  East  and  West  Indies.  The 


446 


THE   DISTRIBUTION   OF   ANIMALS  447 

other  echinoderms  abound  in  almost  every  sea;  the 
starfishes  chiefly  along  the  shore,  the  sea  urchins  in 
the  Littoral  zone,  and  the  sea  slugs  around  coral  reefs. 
Worms  are  found  in  all  parts  of  the  world,  in  sea,  fresh 
water,  and  earth.  They  are  most  plentiful  in  the  muddy 
or  sandy  bottoms  of  shallow  seas.  Living  brachiopods, 
though  few  in  number,  occur  in  tropical,  temperate,  and 
arctic  seas,  and  from  the  shore  to  great  depths.  Poly- 
zoa  have  both  salt  and  fresh  water  forms,  and  annelids 
include  land  forms,  as  the  earthworm  and  some  leeches. 

Mollusks  have  a  world-wide  distribution  over  land  and 
sea.  The  land  forms  are  restricted  by  climate  and  food, 
the  marine  by  shallows  or  depths,  by  cold  currents,  by 
a  sandy,  gravelly,  or  muddy  bottom.  The  bivalves  are 
also  found  on  every  coast  and  in  every  climate,  as  well 
as  in  rivers  and  lakes,  but  do  not  flourish  at  the  depth 
of  much  more  than  two  hundred  fathoms.  The  fresh- 
water mussels  are  more  numerous  in  the  United  States 
than  in  Europe,  and  west  of  the  Alleghanies  than  east. 
The  seashells  along  the  Pacific  coast  of  America  are 
unlike  those  of  the  Atlantic,  and  are  arranged  in  five 
distinct  groups :  Aleutian,  Californian,  Panamic,  Peru- 
vian, and  Magellanic.  On  the  Atlantic  coast,  Cape  Cod 
and  Cape  Hatteras  separate  distinct  provinces.  Of 
land  snails,  Helix  has  an  almost  universal  range,  but  is 
characteristic  of  North  America,  as  Bulimus  is  of  South 
America,  and  Achalina  of  Africa.  The  Old  World  and 
America  have  no  species  in  common,  except  a  few  in 
the  extreme  north. 

The  limits  of  insects  are  determined  by  temperature 
and  vegetation,  by  oceans  and  mountains.  There  is  an 
insect  fauna  for  each  continent,  and  zone,  and  altitude. 
The  insects  near  the  snow  line  on  the  sides  of  mountains 
in  the  temperate  region  are  similar  to  those  in  polar 
lands.  The  insects  on  our  Pacific  slope  resemble  those 


448  COMPARATIVE   ZOOLOGY 

of  Europe,  while  those  near  the  Atlantic  coast  are  more 
like  those  of  Asia.  Less  than  a  score  of  insects  are 
known  to  live  in  the  sea. 

The  distribution  of  fishes  'is  bounded  by  narrower 
limits  than  that  of  other  animals.  A  few  tribes  may  be 
called  cosmopolitan,  as  the  sharks  and  herrings ;  but 
the  species  are  local.  Size  does  not  appear  to  bear  any 
relation  to  latitude.  The  marine  forms  are  three  times 
as  numerous  as  the  fresh-water.  The  migratory  fishes  of 
the  northern  hemisphere  pass  to  a  more  southern  region 
in  the  spring,  while  birds  migrate  in  the  autumn. 

Living  reptiles  form  but  a  fragment  of  the  immense 
number  which  prevailed  in  the  Middle  Ages  of  geology. 
Being  less  under  the  influence  of  man,  they  have  not 
been  forced  from  their  original  habitats.  None  are 
arctic.  America  is  the  most  favored  spot  for  frogs  and 
salamanders,  and  India  for  snakes.  Australia  has  few 
batrachians,  and  two  thirds  of  its  snakes  are  venomous. 
In  the  United  States,  only  about  one  eighth  of  the  species 
are  venomous.  Frogs,  snakes,  and  lizards  occur  at  ele- 
vations of  over  fifteen  thousand  feet.  Crocodiles,  and 
most  lizards  and  turtles,  are  tropical. 

Swimming  birds,  which  constitute  about  one  four- 
teenth of  the  entire  class,  form  one  half  of  the  whole 
number  in  Greenland.  As  we  approach  the  tropics,  the 
variety  and  number  of  land  birds  increase.  Those  of 
the  torrid  zone  are  noted  for  their  brilliant  plumage, 
and  the  temperate  forms  for  their  more  sober  hues,  but 
sweeter  voices.  India  and  South  America  are  the 
richest  regions.  Hummers,  tanagers,  orioles,  and  tou- 
cans are  restricted  to  the  New  World.  Parrots  are 
found  in  every  continent  except  Europe;  and  wood- 
peckers occur  in  every  region,  save  in  Australia. 

The  vast  majority  of  mammals  are  terrestrial;  but 
cetaceans  and  seals  belong  to  the  sea,  otters  and  beavers 


THE   DISTRIBUTION   OF   ANIMALS  449 

delight  in  lakes  and  rivers,  and  moles  are  subterranean. 
As  of  birds,  the  aquatic  species  abound  in  the  polar 
regions.  Marsupials  inhabit  two  widely  separated  areas 
—  America  and  Australia.  Tn  the  latter  continent  they 
constitute  two  thirds  of  the  fauna,  while  nearly  all  placen- 
tal  mammals,  except  bats  and  a  few  rats  and  squirrels, 
are  wanting.  Excepting  a  few  species  in  South  Africa 
and  South  Asia,  edentates  are  confined  to  tropical  South 
America.  The  equine  family  is  indigenous  to  South 
and  East  Africa  and  Southern  Asia,  while  their  fossil 
remains  are  abundant  in  both  North  and  South 
Ameripa.  In  North  America,  rodents  form  about  one 
half  the  number  of  mammals ;  there  are  very  few 
species  in  Madagascar.  Ruminants  are  sparingly 
represented  in  America.  Carnivores  flourish  in  every 
zone  and  continent.  The  prehensile-tailed  monkeys 
are  strictly  South  American ;  while  the  anthropoid  apes 
belong  to  the  west  coast  of  Africa,  and  to  Borneo  and 
Sumatra.  Both  monkeys  and  apes  are  most  abundant 
near  the  equator ;  in  fact,  their  range  is  limited  by  the 
distribution  of  palms. 


CHAPTER   XXV 
THE  ORIGIN   OF  ANIMAL  SPECIES 

THE  origin  of  the  immense  number  of  species  of  plants 
and  animals  inhabiting  the  earth  has  been  a  matter  of 
speculation  among  naturalists  and  philosophers  for  many 
centuries.  One  theory  has  held  that  each  species  was 
created  separately,  while  the  other,  known  as  the  Theory 
of  Evolution,  maintains  that  living  forms  are  derived  by 
natural  processes  of  descent  from  species  that  inhabited 
the  earth  in  earlier  times ;  that  is,  the  ancestral  forms 
became  extinct  owing  to  changing  conditions  of  climate, 
food  supply,  enemies,  and  other  factors,  and  their  de- 
scendants in  the  course  of  many  generations  have  become 
modified  in  bodily  structure  and  function,  these  changes 
leading  to  the  development,  or  evolution,  of  the  numer- 
ous species  now  living.  The  evidence  in  favor  of  the 
latter  theory  is  so  strong  that  it  is  now  accepted  by 
scientific  men  as  the  true  explanation  of  the  mode  of 
origin  of  all  known  organisms. 

Although  the  idea  of  evolution  has  been  more  or  less 
definitely  held  by  various  naturalists  since  the  time  of 
Aristotle  (384-322  B.C.),  others,  even  as  recently  as  Lin- 
naeus (1707- 1778)  and  Cuvier  (1768-1832),  have  insisted 
that  species  are  immutable,  or  unchanging  in  character- 
istics. Bonnet  (1720-1793)  was  the  first  among  later 
zoologists  to  suggest  that  variations  of  climate,  nourish- 
ment, and  other  features  of  the  environment  might  pro- 
duce new  species,  and  to  use  the  term  evolution  in  its 
modern  sense ;  but  he  adduced  no  important  facts  to 

450 


THE   ORIGIN    OF   ANIMAL   SPECIES  451 

support  his  theory,  and  it  failed  to  meet  with  the  ap- 
proval of  his  contemporaries.  Lamarck  (1744-1829) 
afterward  adopted  this  view,  collected  many  facts  in  its 
favor,  and  also  advanced  the  hypothesis,  in  1801,  that 
the  use  and  disuse  of  organs  would  cause  structural 
modifications  in  them,  producing  either  increased  devel- 
opment or  atrophy  of  parts.  These  modifications,  being 
inherited  by  successive  generations,  would  eventually 
become  characteristic  of  new  species  thus  evolved  from 
the  older  ones.  Lamarck's  theory  was  opposed  by 
Cuvier,  the  greatest  comparative  anatomist  and  paleon- 
tologist of  his  time,  who  insisted  that,  if  the  theory  were 
true,  there  ought  to  be  among  fossils  transition  forms 
connecting  the  extinct  with  the  living  species,  but  that 
no  such  forms  were  known,  nor  could  a  process  be  sug- 
gested by  which  transition  could  take  place.  Under 
Cuvier's  leadership  the  belief  became  current  among 
geologists  that  the  earth  has  passed  through  a  series  of 
catastrophes  or  cataclysms  which  destroyed  all  living 
things,  and  that  it  has  successively  been  repeopled  with 
new  forms  quite  unlike  those  which  had  perished.  The 
Lamarckian  theory  passed  into  obscurity,  and  was  not 
seriously  considered  again  until  it  was  brought  forth  for 
comparison  with  Darwin's  theory  of  natural  selection. 
The  opinions  of  geologists  regarding  cataclysms  under- 
went a  change  after  Hutton  (1726-1797)  urged  that  in 
order  to  understand  how  the  present  condition  of  the 
earth  came  about,  the  changes  now  taking  place  must  be 
studied.  This  view  was  later  vigorously  upheld  and  ex- 
tended by  Lyell  (1797-1875),  who  contended  that  cata- 
clysms have  never  occurred,  but  that  the  earth  has 
gradually  reached  its  present  state  through  the  action 
of  natural  forces  which  are  still  in  operation.  Thus  the 
way  was  prepared  for  the  appearance  of  the  theory 
which,  elaborated  and  maintained  by  numerous  observa- 


452  COMPARATIVE   ZOOLOGY 

tions,  was  propounded  by  Charles  Darwin  (1809-1882) 
in  his  "  Origin  of  Species  by  Means  of  Natural  Selec- 
tion, or  the  Preservation  of  Favored  Races  in  the  Strug- 
gle for  Life,"  published  in  1859.  Darwin  had  served  as 
naturalist  on  the  British  exploring  ship  Beagle  on  a  five 
years'  cruise  (1832-1837)  around  the  world,  and  "was 
much  struck  with  certain  facts  in  the  distribution  of  the 
organic  beings  inhabiting  South  America,  and  in  the 
geological  relations  of  the  present  to  the  past  inhabit- 
ants of  that  continent."  After  his  return  home,  twenty 
additional  years  were  spent  in  collecting  facts,  making 
further  observations  and  experiments,  and  in  pondering 
the  theory  before  he  ventured  to  publish  his  results  and 
to  state  what  he  regarded  as  the  factors  concerned  in 
the  process  of  evolution.  A  similar  conclusion  had  been 
reached,  simultaneously  and  independently,  by  Alfred 
Russell  Wallace  (1823-  ),  who  had  travelled  exten- 
sively in  South  America  and  the  Malay  Archipelago, 
and,  like  Darwin,  had  become  convinced  of  the  certainty 
of  evolution,  and  sought  for  its.explanation. 

^As  held  by  Darwin,  the  theory  of  evolution,  together 
with  the  causes  of  the  process,  may  be  briefly  stated  as 
follows :  — 

(i)  Organisms  tend  to  produce  a  great  many  more  off- 
spring than  can  stirvive.  Linnaeus  showed  that  the 
number  of  living  descendants  of  an  annual  plant  which 
produced  only  two  seeds  each  year  would,  at  the  end  of 
twenty  years,  be  over  a  million.  There  is,  however,  no 
plant  known  to  be  so  unproductive.  With  reference  to 
the  elephant,  regarded  as  the  slowest  of  breeders,  pro- 
ducing at  the  age  of  thirty  a  pair  of  young,  and  a  pair 
every  thirty  years  thereafter,  and  living  to  be  one  hundred 
years  old,  Darwin  computed  that  at  the  end  of  750  years 
there  would  be  about  nineteen  million  living  elephants 
all  descended  from  the  first  pair.  Individual  insects  lay 


THE   ORIGIN   OF   ANIMAL   SPECIES  453 

hundreds,  and  fishes  millions,  of  eggs.  If  all  the  young 
were  to  survive,  the  earth  would  soon  be  unable  to  sup- 
ply sufficient  food  and  standing-room. 

(2)  In  spite  of  this  tendency  to  increase  inordinately,  the 
number  of  animals  remains,   on  the  whole,  stationary. 
Even  though  there  may  be  an  enormous  temporary  in- 
crease in  the  number  of  certain  animals,  as  in  "  plagues 
of  grasshoppers,"  normal  conditions  are  soon  restored  by 
natural  agencies.     Eggs  and  young  are  devoured  by  older 
animals.     Disease,  old  age,  parasites,  enemies,  storms, 
floods,  cold,  heat,  drought,  and  famine  are  responsible 
for  the  death  of  so  many  individuals  that  comparatively 
few  young  animals  of  any  species  live  to  maturity. 

(3)  There  results,  consequently,  severe  competition  for 
the  necessaries  of  life,  a  veritable  struggle  for  existence. 
In  order  to  thrive,  animals  need  food,  shelter  from  the 
elements,  protection  from  enemies,  and  freedom  from 
molestation  while  rearing  their  young.     Deprivation  of 
any  of  these  is  likely  to  be  followed  by  serious  results  for 
the  animals  as  individuals  and  for  the  race  as  a  whole. 
The  introduction  of  sheep  has  made  it  impossible  for 
cattle  to  live  on  some  of  the  Western  ranges,  because  the 
sheep  crop  the  grass  so  closely  that  there  is  not  enough 
left  to  feed  the  cattle.     The  "  English,"  or  house,  spar- 
row appropriates  the  best  protected  nesting  places,  raises 
several  broods  each  season,  eats  whatever  food  is  avail- 
able, and  remains  the  year  round  without  migrating.    By 
reason  of  these  habits  it  has  been  victorious  in  the  con- 
test for  the  places  formerly  occupied  by  native  birds. 
The  struggle  for  existence  is  most  keen  between  closely 
related  forms,  since  each  will  naturally  want  what  the 
other  desires.     Until  about  two  centuries  ago  the  black 
rat  was  the  common  rat  of  Europe.     Since  then  it  has 
been  driven  out  by  the  brown  rat,  a  larger  and  stronger 
species. 


454  COMPARATIVE   ZOOLOGY 

(4)  There  is  more  or  less  variation  even  between  closely 
related  animals.  Two  individuals  from  the  same  litter, 
for  instance,  always  differ  somewhat  from  each  other,  as 
well  as  from  all  their  relatives,  in  shape,  size,  vigor,  in- 
telligence, and  other  qualities.  Human  beings,  domes- 
ticated or  wild  animals,  birds,  shells,  or  insects  are  never 
so  much  alike  that  differences  between  individuals  of  the 
same  kind  cannot  be  detected.  Wings  and  tails  of  birds 
of  the  same  species  have  been  found  in  some  individuals 
to  be  twenty  per  cent  longer,  and  in  others  as  much 
shorter,  than  the  average.  Variations,  then,  are  by  no 
means  necessarily  minute,  but  may  be  considerable  in 
amount.  Nor  is  variation  confined  to  structure  alone,  for 
it  may  also  affect  habits.  Chimney  swifts  built  their 
nests  in  hollow  trees  before  the  country  was  settled.  A 
New  Zealand  parrot  which,  before  the  occupation  of  the 
island  by  Europeans,  lived  on  honey,  insects,  and  fruits, 
began  to  pick  at  meat  and  skins  hung  up  to  dry  by  the 
settlers,  and  thus  acquired  a  taste  for  flesh.  During  the 
past  fifty  years  its  carnivorous  propensities  have  increased 
to  such  an  extent  as  to  lead  the  bird  to  attack  living  sheep. 
So  destructive  has  it  become  that  stringent  measures  have 
been  taken  for  its  extermination.  In  animals  under  do- 
mestication variation  is  the  rule.  The  numerous  breeds 
of  cattle,  horses,  swine,  fowls,  pigeons,  dogs,  cats,  rabbits, 
canary  birds,  and  in  fact  of  all  domesticated  animals,  have 
been  derived  from  a  few  ancestral  forms.  Breeders  have 
taken  advantage  of  peculiarities  arising  by  variation  ;  and 
by  a  process  of  selecting  and  breeding  only  from  those 
individuals  which  show  the  peculiarity,  have  finally  suc- 
ceeded in  fixing  it  more  or  less  permanently,  so  that  the 
young  of  these  animals  may  possess  it  in  an  even  more 
exaggerated  form  than  their  parents.  In  nature  varia- 
tions occur  to  such  an  extent,  particularly  in  large  and 
dominant  genera,  as  to  give  rise  to  many  doubtful  species, 


THE   ORIGIN   OF   ANIMAL   SPECIES  455 

or    forms    which    are    intermediate     between     typical 
species. 

The  causes  of  variability  are  at  present  very  imper- 
fectly understood,  but  it  is  probable  that  climate,  nourish- 
ment, and  physiological  activity,  as  well  as  other  factors, 
have  an  influence  on  the  process. 

(5)  Even   though  animals  may  be  inclined  to  vary, 
there  is  a  marked  tendency  to  inherit  the  characteristics 
of  their  parents.       Every    animal    bears   a    close    re- 
semblance  to   others   of   its   kind.     It   is   due   to  this 
tendency  that   structural   and  physiological  character- 
istics once  originated  are  perpetuated.      The  breeder 
depends  upon  it  to  keep  his  varieties   "  true  "  to  the 
original   stock.      Whether  or   not   characters  acquired 
during  the  lifetime  of  an  individual  are  transmitted  by 
heredity  to  the  offspring  is  still  an  unsettled  problem. 
Denial  of  the  probability  of  such  inheritance  is  a  fun- 
damental theory  in  the  Weismannian  school  of  evolu- 
tionists.    There  seems  to  be  no  doubt,  however,  that 
congenital  characters  are  inherited.     Should  variations 
appear,  they  are  likely  to  be  preserved  to  the  race  by 
heredity.     The  essential  nature  of  the  process,   as  in 
variation,  is  not  known.      To  explain  the  phenomena  of 
heredity,   Darwin   proposed  the  theory   of  pangenesis, 
which  holds  that  particles  or  gemmules  from  all  the 
different  parts  of  the  body  are  collected  into  the  repro- 
ductive cells  and  through  these  are  transmitted  to  the 
offspring  and  help  to  modify  the  characteristics  of  the 
latter.     This  theory  has  never  been  generally  accepted. 

(6)  The  preservation  or  survival  of  those  individuals 
inheriting  the  variations  which  are  most  advantageous  in 
the   struggle  for  existence  is  due   to   natural  selection. 
Among  all  the  variations  appearing  in  the  individuals 
of  a  race  some  are  likely  to  be  advantageous,  others  the 
opposite.     An  antelope  with  slightly  longer  legs  or  with 


456  COMPARATIVE   ZOOLOGY 

more  agility  than  other  members  of  the  herd  would  be 
better  able  to  escape  his  enemies,  and  consequent^  to 
live  longer  and  leave  more  offspring  than  his  less 
fortunate  companions.  Of  his  progeny  some  would 
probably  inherit  the  peculiarity,  and  it  would  thus  be 
transmitted  from  generation  to  generation  to  the  evi- 
dent advantage  of  the  race.  In  time  the  variation 
would  become  a  definitely  fixed  and  constant  character, 
serving  to  distinguish  all  the  individuals  possessing  it  as 
a  species.  It  is  by  a  process  of  artificial  selection  that 
breeders  choose  among  domestic  animals  those  indi- 
viduals which  possess  a  character  which  it  is  desired  to 
perpetuate,  as  long  wool  in  sheep,  speed  in  race  horses, 
strength  in  draught  horses,  peculiarly  shaped  jaws  in 
bull  dogs,  vocal  powers  in  canary  birds,  and  so  on.  By 
breeding  only  from  those  individuals  which  show  the 
desired  character,  the  latter  may  not  only  be  perpetu- 
ated but  also  intensified  in  degree.  This  is  shown  by 
all  domesticated  animals  and  cultivated  plants.  To  the 
process  by  which  favorable  variations  are  selected  and 
perpetuated  among  wild  animals,  Darwin  gave  the  name 
of  natural  selection.  By  his  hypothesis  the  phenomenon 
of  the  evolution  of  organic  forms  is  due  to  the  natural 
selection  of  favorable  variations  and  their  preservation 
by  heredity. 

The  test  of  the  validity  of  a  theory  lies  in  its  ability 
to  interpret  and  coordinate  observed  facts,  and  when 
Darwin's  hypothesis  was  applied  to  the  elucidation  of 
the  observations  collected  by  the  students  of  morphol- 
ogy, paleontology,  embryology,  and  other  aspects  of 
the  study  of  animal  life,  it  brought  order  out  of  chaos, 
and  each  of  these  sciences  was  seen  to  contain  a  mass 
of  evidence  in  favor  of  the  theory  of  descent  with 
modification. 

(i)  Evidence  from  Classification.  —  As  has  been  shown 


THE   ORIGIN    OF   ANIMAL   SPECIES  457 

in  Part  I,  animals  are  divided  according  to  their  struc- 
tural resemblances  into  groups  of  varying  degrees  of 
affinity,  as  branch,  class,  order,  genus,  and  species.  A 
pictorial  representation  of  the  scheme  of  classification 
would  have  the  form  of  a  genealogical  tree  (Fig.  197), 
the  relative  positions  of  whose  branches  would  indicate 
the  degree  of  relationship  among  the  different  groups. 
This  scheme  implies  that  there  is  actual  genetic  relation- 
ship and  community  of  descent  of  all  animal  forms,  the 
Metazoa  from  the  Protozoa,  the  air-breathing  vertebrates 
from  fishlike  ancestors,  the  birds  and  mammals  from 
reptilian  prototypes.  There  is  thus  evolution  of  the 
more  complex  from  forms  of  simpler  structure.  The 
underlying  principle  of  classification  is  heredity,  or 
community  of  descent,  as  indicated  by  family  likeness. 
After  trying  many  other  structural  features  as  means  of 
classification  systematic  zoologists  found  that  the  surest 
guides  in  determining  relationship  are  frequently  organs 
of  little  or  no  assignable  physiological  importance. 
There  was  no  explanation  of  this  seeming  paradox 
until  it  was  seen  that,  according  to  Darwin's  theory, 
such  organs  are  not  likely  to  undergo  change,  since  they 
are  apparently  not  of  vital  importance  to  the  possessor 
and  are  handed  down  through  successive  generations 
little,  if  at  all,  modified,  however  much  the  rest  of  the 
body  may  have  changed  in  becoming  adapted  to  its 
environment. 

(2)  Evidence  from  Morphology.  —  The  comparative 
anatomy  of  animals  furnishes  some  of  the  strongest 
evidence  in  favor  of  the  theory  of  evolution.  Note,  for 
instance,  the  increasing  complexity  of  form  and  function 
as  the  various  branches  are  passed  in  review  —  the  single- 
celled  Protozoa,  showing  colony  formation  in  the  higher 
orders  with  differentiation  in  form  and  function  among  the 
members,  as  Zoothamnium  ;  the  loosely  cellular  sponges, 


458  COMPARATIVE  ZOOLOGY 

the  lowest  of  the  Metazoa,  long  considered  to  be  col- 
onies of  Protozoa,  so  ill  defined  are  their  layers  of  tissue; 
the  two-layered  coelenterates  whose  bodies  contain  but 
a  single  cavity  with  one  opening  serving  for  ingestion 
and  egestion;  the  "worms,"  with  a  body  consisting, 
except  in  some  of  the  parasitic  and  degenerate  forms, 
of  three  layers  of  tissue  (ectoderm,  mesoderm,  and 
endoderm),  with  an  alimentary  canal  having  both  inlet 
and  outlet,  a  well-defined  nervous  system,  and,  in  the 
higher  orders,  a  segmented  body ;  the  arthropods,  with 
their  segments  showing  a  tendency  to  become  grouped 
into  distinct  regions,  with  jointed  appendages  for  per- 
forming functions,  and  with  respiratory  organs,  in  the 
higher  groups,  for  breathing  air  directly ;  the  verte- 
brates, beginning  with  forms  which  have  the  merest 
trace  of  a  notochord,  and  progressing  through  the  lower 
fishes  with  cartilaginous  skeletons,  small  brains,  and 
two-chambered  hearts,  to  the  amphibia  and  reptiles,  with 
bony  skeletons,  larger  brains,  and  three-chambered 
hearts,  and  finally  to  the  warm-blooded  birds  and  mam- 
mals, with  four-chambered  hearts,  and  with  brains  and 
nervous  systems  far  superior  in  size,  structure,  and 
function  to  those  of  all  other  groups.  This  hasty 
review  does  not,  by  any  means,  take  cognizance  of  all 
the  structural  features  that  might  be  mentioned,  but 
only  draws  attention  to  some  of  the  most  obvious  char- 
acters which  show  progressive  change  from  lower  to 
higher  forms. 

The  metameric  arrangement  of  the  bodies  of  the 
annulata,  in  which  each  segment  is  a  more  or  less  per- 
fect repetition  of  the  preceding  and  succeeding  seg- 
ments, is  again  recognizable,  though  less  plain,  in  certain 
structures  in  the  bodies  of  crustaceans  and  insects,  and 
is  very  obvious  in  the  chordates  from  fishes  to  man,  as 
the  vertebrae  and  pairs  of  ribs,  the  muscle  plates  and 


THE   ORIGIN   OF   ANIMAL   SPECIES  459 

pairs  of  muscles,  the  spinal  nerves  and  ganglia,  the 
intercostal  arteries  and  veins. 

A  comparison  of  such  dissimilar  organs  as  the  wing 
of  the  bat  and  the  bird,  the  flipper  of  the  seal,  the  pec- 
toral fin  of  the  fish,  the  hoof  of  the  horse,  and  the  hand 
of  man  shows  evidence  of  genetic  relationship  in  that  all 
are  constructed  on  one  fundamental  plan,  which  has 
been  modified  to  meet  the  needs  of  different  environ- 
ments. 

The  testimony  of  rudimentary  organs  is  also  in  favor 
of  the  theory  of  descent  with  modification.  The  embryo 
of  the  whalebone  whale  has  teeth,  but  they  never  cut 
the  gum.  Their  presence  is  explainable  only  on  the 
hypothesis  that  this  animal  is  a  descendant  of  some  form 
that  had  functional  teeth.  Nearly  half  of  the  beetles 
inhabiting  the  wind-swept  island  of  Madeira  have  such 
rudimentary  wings  that  flight  is  impossible,  though  there 
is  no  doubt  that  these  insects  were  once  capable  of  fly- 
ing, since  the  nearest  related  species  which  live  on  the 
mainland  have  fully  developed  wings.  In  this  case, 
inability  to  fly  is  a  distinct  advantage,  because  it  renders 
the  insect  less  likely  to  be  blown  out  to  sea  and  drowned. 
The  presence  of  rudimentary  and  functionless  eyes  in 
cave-inhabiting  animals  indicates  descent  from  ancestors 
having  perfect  visual  organs. 

(3)  Evidence  from  Embryology.  —  Of  the  many  im- 
portant facts  which  this  branch  of  science  offers  in 
support  of  the  theory  of  evolution,  only  a  few  can  be 
mentioned  here.  It  has  been  learned  that  higher  ani- 
mals in  the  course  of  their  embryonic  development  pass 
through  stages  which  are  permanent  conditions  in  lower 
forms.  Thus  the  bird  and  the  mammal,  though  they 
never  possess  gills  in  their  adult  life,  have  at  an  early 
stage  of  existence  a  series  of  openings,  gill  slits,  in  the 
side  of  the  neck,  corresponding  to  the  gill  openings  of 


460  COMPARATIVE   ZOOLOGY 

the  fish,  with  a  system  of  blood  vessels  similar  to  those 
in  the  fish's  gills.  These  openings  afterward  close  and 
disappear,  and  the  most  of  the  blood  vessels  waste  away. 
Thus,  a  condition  which  is  permanent  in  the  fish  is  only 
transitory  in  the  higher  vertebrates,  and  it  can  be  ex- 
plained only  on  the  supposition  that  during  their  devel- 
opment these  forms  repeat  the  phases  through  which 
their  ancestors  passed  in  the  course  of  their  evolution. 
Embryology  thus  corroborates  paleontology  in  showing 
that  the  earliest  vertebrates  were  fishlike.  All  of  the 
vertebrates  possess  at  a  certain  period  of  their  embryonic 
life  a  notochord  or  rod  of  cartilage,  which  is  later  re- 
placed by  a  vertebral  column  in  all  forms  except  Amphi- 
oxus,  in  which  the  organ  persists.  This  indicates  that 
vertebrates  are  descended  from  an  amphioxus-like  ances- 
tor. Until  their  embryological  history  was  learned, 
Ascidians  were  considered  to  be  mollusks.  The  dis- 
covery of  a  rudimentary  notochord  at  an  early  stage  of 
their  development  showed  that  they  are  closely  allied  to 
the  vertebrates.  The  adult  halibut,  turbot,  sole,  and 
other  "flat  fish  "  have  both  eyes  on  the  same  side  of  the 
head.  In  the  embryonic  stage  the  eyes  are  placed  as 
in  other  fishes,  showing  that  in  their  ancestors  the  eyes 
were  in  the  usual  position.  A  West  Indian  frog,  Hylo- 
des,  which  lays  eggs  on  land,  passes  through  its  tadpole 
stage  in  the  egg.  It  has  a  large  tail,  like  the  ordinary 
tadpole,  and  gill  slits  in  place  of  functional  gills.  The 
tail  wastes  away  almost  entirely,  and  lungs  are  devel- 
oped before  hatching  occurs,  the  young  animal  thus 
entering  upon  its  terrestrial  mode  of  life  unhampered  by 
organs  suited  only  to  an  aquatic  existence.  Its  develop- 
ment shows  that  it  is  descended  from  ancestral  forms 
which  had  both  tail  and  gills. 

The  tendency  of  animals  to.  pass  through  stages  of 
development  in  which  they  temporarily  exhibit  features 


THE   ORIGIN   OF   ANIMAL   SPECIES  461 

which  are  permanent  characteristics  of  forms  lower  than 
themselves,  is  the  basis  of  the  Recapitulation  Theory, 
which  holds  that  each  animal  bears  the  marks  of  its  own 
ancestry  and  reveals  its  parentage  in  its  own  develop- 
ment. 

(4)  Evidence  from  Paleontology.  —  Although  only  a 
very  small  part  of  the  earth's  crust  has  been  examined, 
the  fossil  animal  remains  already  found  furnish  the  pri- 
mary and  most  direct  evidence  in  favor  of  the  theory  of 
descent  with  modification,  and  they  show  that  the  pro- 
cess began  as  far  back  in  geological  history  as  they  can 
be  traced.  The  conclusions  reached  from  the  study  of 
fossils  may  be  stated  in  the  words  of  the  great  paleon- 
tologist Zittel :  (i)  All  stratified  sedimentary  rocks 
(with  the  exception  of  metamorphic  rocks)  inclose,  more 
or  less  richly,  fossils,  and  thus  prove  that  the  earth,  for 
an  immeasurable  length  of  time  before  the  appearance 
of  man,  was  inhabited  by  organisms.  (2)  Fossils  of  the 
oldest  and  deepest  strata  represent  extinct  species,  and 
for  the  most  part  extinct  genera;  only  in  the  more 
recent  strata  are  found  forms  which  are  identical  with 
those  now  living.  The  deeper  down  we  penetrate  in 
the  series  of  strata,  the  more  divergent  are  the  fossils 
from  the  forms  now  living ;  and,  on  the  contrary,  rising 
from  the  earliest  to  the  more  recent  formations  there 
is  a  continuously  increasing  resemblance  to  the  present 
creation.  (3)  The  different  fossil  faunas  and  floras 
follow  each  other  the  world  over  in  the  same  regular 
sequence ;  the  formations  stratigraphically  nearer  to 
each  other  contain  the  most  similar  fossils,  and  those 
most  separated  in  age  present  the  greater  differences. 
(4)  Constant  change  characterizes  the  evolution  of  the 
organic  creation.  Species  of  one  geological  formation 
are  either  completely  or  partly  replaced  by  other  species 
in  the  next  superimposed  strata.  (5)  Each  species,  like 


462  COMPARATIVE   ZOOLOGY 

the  individual,  has  a  certain  shorter  or  longer  life  period, 
after  which  it  perishes,  never  to  reappear.* 

The  genealogical  history,  or  line  of  descent,  of  several 
animals  has  been  very  completely  established  since 
fossils  came  to  be  studied  in  the  light  of  the  theory 
of  evolution.  The  ancestry  of  the  horse  has  been 
traced  through  forms  which  follow  one  another  in 
linear  series  from  remote  geological  periods.  The 
earliest  form  was  about  as  large  as  a  sheep  and  had 
five  toes  and  short  molar  teeth, '  with  a  comparatively 
simple  arrangement  of  the  ridges  on  their  crowns. 
This  was  succeeded  by  four-toed,  three-toed,  and  eventu- 
ally the  single-toed  horse  of  modern  times.  The  dimi- 
nution in  the  number  of  the  digits  was  accompanied  by 
gradually  increasing  stature  and  growing  complexity  of 
the  crown  patterns  of  the  teeth.  An  even  more  com- 
plete series  of  remains  from  the  bed  of  an  ancient 
lake  establishes  the  genealogy  of  the  fresh-water  snail, 
Planorbis.  "  In  passing  from  the  lowest  to  the  highest 
strata  the  species  change  greatly  and  many  times,  the 
extreme  forms  being  so  different  that,  were  it  not  for 
the  intermediate  forms,  they  would  be  called  not  only 
different  species,  but  different  genera.  And  yet  the 
gradations  are  so  insensible  that  the  whole  series  is 
nothing'  less  than  a  demonstration,  in  this  case  at  least, 
of  origin  of  species  by  derivation  with  modifications. "f 
Other  series  show  the  evolution  of  the  horn-bearing 
ruminants  from  hornless  ancestors.  Casts  of  the  brain 
cavities  of  the  early  mammals  show  that  their  brains 
were  much  smaller  than  those  of  living  species  and  had 
fewer,  if  any,  convolutions. 

Of  the  "  missing  links  "  none  is  more  instructive  than 

*  Quoted  from  Williams's  "  Geological  Biology,"  pp.  82-83. 
t  Quoted  from  Le  Conte's  "Evolution  and  its  Relations  to  Religious 
Thought,"  pp.  254-255. 


THE   ORIGIN    OF   ANIMAL   SPECIES  463 

Archceopteryx,  which  occupies  a  position  between  the 
reptiles  and  the  birds.  Its  fossil  remains  show  its 
reptilian  characters  in  the  separate  digits  on  the  fore  limb, 
the  elongated  tail  consisting  of  many  vertebrae,  and  the 
well-developed  teeth  in  each  jaw.  Its  most  prominent 
avian  features  were  its  wings  and  covering  of  feathers. 
The  discovery  of  extinct  toothed  birds  served  to  make 
the  connection  between  birds  and  reptiles  complete. 

(5)  Evidence  from  Geographical  Distribution.  — When 
the  faunas  of  the  different  continents  are  compared, 
they  are  found  to  be  very  unlike.  Even  in  those  re- 
gions which  have  much  the  same  climate  and  other 
physical  conditions,  as  South  Africa,  South  America, 
and  Australia,  the  faunas  are  not  correspondingly  simi- 
lar. On  the  other  hand,  when  the  animals  inhabiting 
the  northern  part  of  South  America  are  compared  with 
those  living  in  the  southern  portion,  there  are  found  to 
be  closer  resemblances  than  in  the  instances  just  noted, 
even  though  the  climatal  differences  are  much  greater. 
A  similar  statement  could  be  made  regarding  other 
great  continental  areas.  There  is  no  native  species 
of  mammal  common  to  Europe,  America,  and  Australia, 
though  introduced  species  thrive.  Rabbits,  for  example, 
taken  from  Europe  to  Australia  have  multiplied  to  such 
an  extent  as  to  have  become  veritable  pests.  Evidently 
differences  of  climate  do  not  alone  account  for  the  pres- 
ent geographical  distribution  of  animals.  Great  barriers, 
as  oceans,  lofty  mountain  ranges,  and  deserts,  separate 
faunas,  though  the  differences  are  not  so  great  as  in  the 
case  of  distinct  continents.  Again,  while  it  is  noted  that 
different  regions  of  a  continent  are  inhabited  by  distinct 
species,  it  is  found  that  these  species  are  more  nearly 
related  among  themselves  than  to  the  species  of  other 
continents.  For  instance,  the  humming  birds,  near  rela- 
tives of  the  sunbirds  of  Africa  and  Asia,  number  about 


464  COMPARATIVE   ZOOLOGY 

four  hundred  species,  and  are  all  confined  to  the  Western 
Hemisphere.  The  explanation  of  this  fact  is  that  they 
originated  in  this  part  of  the  world,  and  are  too  small 
and  weak  to  make  the  long  flight  necessary  to  reach 
other  regions. 

Islands  are  usually  populated  by  forms  brought  from 
the  nearest  mainland,  unless  the  ocean  currents  are  such 
as  to  bring  animals  from  places  more  remote.  Such 
islands  as  are  separated  from  a  neighboring  continent 
by  deep  channels  have  a  fauna  more  archaic  and  primi- 
tive than  that  found  on  the  mainland.  Australia  and  New 
Zealand  are  thought  to  have  been  separated  from  the 
nearest  larger  bodies  of  land  for  long  geological  periods, 
and  possibly  since  the  time  of  their  formation.  Their 
faunas  are  of  a  very  primitive  type,  including  the  mar- 
supials, one  of  the  oldest  and  least  highly  developed 
orders  of  mammals;  the  monotremes,  Ornithorhynchus 
and  Echidna,  the  lowest  representatives  of  the  same 
class,  and  Apteryx,  the  lowest  of  living  birds.  Isolated 
on  their  island  continents  and  free  from  the  competition 
of  higher  forms,  and  especially  from  the  attacks  of  car- 
nivores, these  lowly  organized  and  almost  defenseless 
species  have  retained  to  a  marked  degree  the  character- 
istics of  their  remote  ancestors.  Where  the  separation 
of  island  and  continent  has  taken  place  more  recently, 
or  where  the  channel  is  shallow  or  narrow,  there  is 
greater  resemblance  between  their  faunas.  Thus,  wild 
animals  of  Great  Britain  are  quite  the  same  as  those 
of  western  Europe.  The  number  of  species  found  on 
islands  is  usually  small  as  compared  with  those  inhabit- 
ing an  equal  continental  area,  because  the  number  of 
ancestral  forms  which  have  been  carried  to  the  island  by 
currents,  wind,  and  man  is  likely  to  be  small. 

The  animals  found  on  high  mountain  peaks  and  ranges 
are  distinctly  allied  to  arctic  forms.  On  the  northward 


THE   ORIGIN   OF   ANIMAL   SPECIES  465 

retreat  of  the  great  ice  sheet  which,  during  the  last  gla- 
cial period,  covered  much  of  Europe,  Asia,  and  North 
America,  these  boreal  species  were  left  stranded  in,  and 
have  since  been  confined  to,  regions  having  arctic  char- 
acteristics. Species  which  are  closely  related  to  one 
another  are  known  to  inhabit  mountain  peaks,  separated 
by  long  stretches  of  lowland,  on  which  animals  of  the 
arctic  type  could  not  possibly  exist.  Their  presence 
can  be  explained  only  on  the  supposition  that  they  are 
descended  from  forms  which  inhabited  the  entire  region 
during  the  glacial  period  and  that  when  the  climate 
became  warmer  these  animals  retreated  to  the  cold 
mountain  tops.  Many  oceanic  islands  are  destitute  of 
batrachians  and  terrestrial  mammals,  these  animals  not 
having  had  an  independent  evolution  in  these  localities, 
nor  being  able  to  make  their  way  out  from  the  mainland. 
On  the  other  hand,  aerial  mammals,  as  bats,  are  of 
nearly  universal  distribution. 

It  is  generally  admitted  that  each  species  originated  in 
one  locality  and,  by  migration,  spread  into  neighboring 
regions,  becoming  modified  as  dispersal  brought  it  into 
different  environments,  thus  giving  rise  to  variations 
which  ultimately  resulted  in  the  development  of  a  num- 
ber of  more  or  less  closely  related  species. 

Such  are  the  main  features  of  the  theory  of  evolution 
and  of  Darwin's  explanation  of  the  process  through  va- 
riation, heredity,  and  natural  selection.  As  a  subordinate 
factor  should  be  mentioned  his  theory  of  sexual  selection, 
by  which  is  meant  that  the  choice  of  mates,  among  the 
higher  animals  at  least,  is  largely  determined  by  such 
physical  characteristics  as  strength,  beauty  of  form, 
coloration,  and  vocal  powers.  Those  individuals,  for 
instance,  which  possessed  any  of  these  pleasing  char- 
acteristics in  a  higher  degree  than  their  companions 
would  be  more  likely  to  find  mates  and  to  leave  de- 
DODGE'S  GEN.  ZOOL.  —  30 


466  COMPARATIVE   ZOOLOGY 

scendants.  On  this  theory  Darwin  accounted  for  the 
development  of  antlers,  the  beautiful  colors  of  birds, 
fishes,  and  insects,  and  the  calls  of  various  animals. 

While  evolution  has  come  to  be  regarded  as  a  fact  of 
as  much  certainty  as  gravitation,  and  natural  selection, 
with  variation  and  heredity,  to  be  accepted  by  many 
naturalists  as  the  process  by  which  evolution  is  brought 
about,  not  all  are  agreed  as  fo  the  importance  to  be 
attributed  to  the  Darwinian  factor,  i.e.  natural  selection. 
Darwin  himself  regarded  it  as  "the  main  but  not  the 
exclusive  means  of  modification."  Search  for  additional 
means  has  been  made  and  is  still  being  prosecuted.  Thus 
consciousness  is  claimed  to  be  a  controlling  agent  in  the 
use  and  disuse  of  organs,  in  the  adoption  of  new  habits, 
in  the  selection  of  environment,  in  the  choice  of  mates, 
and  so  on.  Isolation  due  to  the  geographical  separation 
of  individuals  or  of  species,  or  to  the  inability  of  forms 
to  interbreed  {physiological  isolation)  has  been  suggested 
as  another  cause  of  modification.  Still  another  factor, 
organic  selection,  has  been  proposed.  It  is  claimed  for 
this  that  the  adoption  of  a  new  habit  by  an  animal  will 
lead  to  the  development  of  structures  adapted  to  the 
habit,  and  thus  produce  changes  in  specific  differences. 

The  most  important  addition  to  the  philosophy  of 
organic  evolution  made  since  Darwin  is  Weismann's 
theory  of  the  continuity  of  the  germ  plasm,  which 
maintains,  supported  by  facts  of  observation,  that  the 
essential  germinal  substance  is  transmitted  from  gen- 
eration to  generation  through  the  reproductive  cells. 
Whether  or  not  this  material  —  the  bearer  of  heredity 
—  may  be  influenced  by  structural  and  physiological 
changes  occurring  in  the  species,  and  thus  be  trans- 
mitted to  descendants,  is  a  question  which  has  not  yet 
been  definitely  answered. 


NOTES 


1  The  complete  and  elaborate  natural  history  of  a  single  species  or  lim- 
ited group  is  called  a  Monograph,  as  Darwin's  "  Monograph  of  the  Cirri- 
pedia."     A  Memoir  is  not  so  formal  or  exhaustive,  giving  mainly  original 
investigations  of  a  special  subject,  as  Owen's  "  Memoir  on  the  Gorilla." 

2  Before  the  time  of  Linnaeus,  the  ladybug,  e.g.,  was  called  "the  Cocci- 
nella  with  red  coleopters  having  seven  black  spots."     He  called  it   Cocci- 
nella  septem-punctata. 

8  Mondino  (1315)  and  Berenger  (1518)  of  Bologna,  and  Vesalius  of 
Brussels  (1543),  were  the  first  anatomists.  Circulation  of  the  blood  dis- 
covered by  Harvey,  1616.  The  lacteals  discovered  by  Asellius,  1622,  and 
the  lymphatics  by  Rudbek,  1650.  Willis  made  the  first  minute  anatomy 
of  the  brain  and  nerves,  1659.  The  red  blood  corpuscles  were  discovered 
by  Swammerdam,  1658.  Infusoria  first  observed  by  Leeuwenhoek,  1675; 
the  name  given  by  M  tiller,  1786.  Swammerdam  was  the  founder  of 
Entomology,  1675.  Comparative  anatomy  was  first  cultivated  by  Perrault, 
Pecquet,  Duverney,  and  Mery,  of  the  Academy  of  Paris,  the  latter  part 
of  the  seventeenth  century.  Malpighi,  the  founder  of  structural  anatomy, 
was  the  first  to  demonstrate  the  structure  of  the  lungs  and  skin,  1661. 
About  the  same  time,  Ray  and  Willoughby  first  classified  fishes  on  struc- 
tural grounds.  Foraminifers  were  seen  by  Beccarius  one  hundred  and 
fifty  years  ago;  but  their  true  structure  was  not  demonstrated  till 
r835>  by  Dujardin.  Peyssonel  published  the  first  elaborate  treatise  on 
Corals,  1727.  Haller  was  the  first  to  distinguish  between  contractility 
and  sensibility,  1739.  White  blood  corpuscles  discovered  by  Hewson  in 
1775.  Spallanzani  was  the  first  to  demonstrate  the  true  nature  of  the 
digestive  process,  1777.  Cuvier  and  Geoffrey,  in  1797,  proposed  the  first 
natural  classification  of  animals.  Before  that,  all  invertebrates  were  di- 
vided into  insects  and  worms.  Lamarck  was  the  first  to  study  mollusks, 
1800;  before  him,  attention  was  confined  to  the  shell.  He  separated 
spiders  from  insects  in  1812.  The  law  of  correlation  enunciated  by  Cuvier, 
1826.  Von  Baer  was  the  founder  of  Embryology,  establishing  the  doctrine 
omnia  ex  ovo,  1827;  but  the  first  researches  in  Reproduction  were  made 
by  Fabricius  about  1600,  and  by  Harvey  in  1651.  Wolff,  in  the  i8th  cen- 
tury, was  the  pioneer  in  observing  the  phenomena  of  Development.  Sars 
first  observed  alternate  generation,  1833.  Dumeril  is  considered  the 

467 


468  NOTES 

father  of  Herpetology,  and  Owen  of  Odontology.  Schleiden  and  Schwann 
published  their  celebrated  researches  in  cell  structure,  1841;  but  Bichat, 
who  died  1802,  was  the  founder  of  Histology.  Protoplasm  was  discovered 
by  Dujardin  in  1835,  anc^  called  Sarcode.  The  name  Protoplasma  was 
formally  given  to  the  slimy  contents  of  vegetable  cells  by  the  German  bot- 
anist, Hugo  von  Mohl,  in  1846.  The  essential  identity  of  the  protoplasm 
of  plants  and  of  animals  was  first  claimed  by  Max  Schulze  in  1861,  who 
thus  made  one  of  the  most  important  generalizations  in  science. 

4  According  to  Mr.  Darwin,  the  characters  which  naturalists  consider 
as  showing  true  affinity  between  two  or  more  species  are  those  which  have 
been  inherited  from  a  common  parent ;   and,  in  so  far,  all  true  classification 
is  genealogical,  i.e.,  it  is  not  a  mere  grouping  of  like  with  like,  but  it  in- 
cludes, like  descent,  the  cause  of  similarity.     In  the  existing  state  of  science 
a  perfect  classification  is  impossible,  for  it  involves  a  perfect  knowledge  of 
all  animal  structure  and  life  history.     As  it  is,  it  is  only  a  provisional  at- 
tempt to  express  the  real  order  of  nature,  and  it  comes  as  near  to  it  as  our 
laws  do  in  explaining  phenomena.     It  simply  states  what  we  now  know 
about  comparative  anatomy  and  physiology.     As  science  grows,  its  lan- 
guage will  become  more  precise  and  its  classification  more  natural. 

5  The  term  type  is  also  used  to  signify  that  form  which  presents  all  the 
characters  of  the   group   most    completely.     Each  genus  has  its  typical 
species,  each  order  its  typical  genus,  etc.     The  word  is  also  applied  to 
the  specimen  on  which  a  new  species  is  founded.     A  persistent  type  is  one 
which  has  continued  with  very  little  change  through  a  great  range  of  time. 
The  family  of  oysters  has  existed  through  many  geological  ages. 

6  The  Coelenterata  and  Echinodermata  together  make  up  the  Radiata, 
the  old  subkingdom  of  Cuvier.     Echinoderma  is  probably  more  correct - 
than  Echinodermata^  but  we  retain  the  old  orthography. 

7  Strictly  speaking,  no  individual  is  independent.     Such  is  the  division 
of  labor  in  a  hive,  that  a  single  bee,  removed  from  the  community,  will 
soon  die,  for  its  life  is  bound  up  with  the  whole.     An  individual  repeats 
the  type  of  its  kingdom,  branch,  class,  order,  family,  genus,  and  species, 
through  its  whole  line  of  descent. 

8  These  definitions  of  the  various  groups  are  mainly  taken  from  Agassiz. 
They  are  not  practically  very  useful,  as  they  are  not  used  by  working  natu- 
ralists.    The  kind  and  degree  of  difference  entitling  a  group  to  a  particular 
rank  varies  greatly  with  the  naturalist,  and  the  part  of  the  animal  kingdom 
where  the  group  is  found.     Some  families  of  insects  are  separated  by  gaps 
less  than  those  which  divide  genera  of  mammals. 

9  The  millepore  coral,  so  abundant  in  the  West  Indian  Sea,  is  the  work 
of  hydroids.     The  surface  is  nearly  smooth,  with  minute  punctures.     Ge- 
genbaur,  Haeckel,  and  others  hold  that  the  acalephs  have  no  body  cavity 
at  all,  the  internal  system  of  canals  being  homologous  with  the  intestinal 
cavity  of  other  animals. 


NOTES  469 

10  This  digestive  cavity  is  really  homologous  to  the  proboscis  of  the 
jellyfish,  turned  in.     It  is  lined  with  ectoderm.     The  "body  cavity"  is 
not  really  such,  but  is  homologous  to  the  digestive  sac  of  the  hydra. 

11  Among  the  exceptions  are  Tubipora,  which  have  eight  tentacles  and 
no  septa,  and  the  extinct  Cyathophylla,  whose  septa  are  eight  or  more. 

12  The  longer  septa  (called  primary)  are  the  older;   the  shorter,  sec- 
ondary ones  are  developed  afterward.     As  a  rule,  sclerodermic  corals  are 
calcareous,  and  a  section  is  starlike;   the  sclerobasic  are  horny  and  solid. 
The  latter  are  higher  in  rank. 

13  The  most  important  genera  are  Terebratula,  Rhynchonella,  Discina, 
Lingula,  Orthis,  Spirifera,  and  Productus.     The  first  four  have  represen- 
tatives in  existing  seas.     Most  naturalists  now  admit  their  affinity  to  the 
worms,  some  still  keep  them  in  the  branch  Mollusca,  while  others  include 
them  in  the  separate  branch  Molluscoida. 

14  Some  starfishes    {Solaster}  have  twelve  rays.     In  all  echinoderms, 
probably,  sea  water  is  freely  admitted  into  the  body  cavity  around  the 
viscera. 

15  The  shell  is  not  strictly  external,  like  the  crust  of  a  lobster,  but  is 
covered  by  the  external  skin. 

16  Six  hundred  pieces  have  been  counted  in  the  shell  alone,  and  twelve 
hundred  spines.     The  feet  number  about  eighteen  hundred.     They  can 
be  protruded  beyond  the  longest  spines. 

.  17  Certain  crabs  live  on  dry  land,  but  they  manage  to  keep  their  gills 
wet. 

18  The  student  should  remember  that   this   threefold   division  is   not 
equivalent  to  the  like  division  of  a  vertebrate  body. 

19  Each  ring  (called  somite)  is  divisible  into  two  arcs,  a  dorsal   and 
ventral. 

20  The  eye  stalks  were  formerly  considered  to  be  appendages,  but  are 
no  longer  so  regarded. 

21  These   parts   do   not  correspond  to  the  parts  so  named  in   human 
anatomy.     See  also  pp.  371,  372. 

22  The  four  pairs  of  legs  in  arachnids  answer  to  the  third  pair  of  max- 
illge  and  the  three  pairs  of  maxillipedes  in  the  lobster.     The  great  claws  of 
scorpions  and  the  pedipalpi  of  spiders  correspond  to  the  first  maxillae  of 
the  lobster. 

23  Compare  the  single  thread  of  the  silkworm  and  other  caterpillars. 

24  The  common  spider,  Epeira,  which  constructs  with  almost  geometri- 
cal precision  its  net  of  spirals  and  radiating  threads,  will  finish  one  in  forty 
minutes,  and  just  as  regularly  if  confined  in  a  perfectly  dark  place. 

25  There  are  some  exceptions :  the  oyster  is  unequivalved,  and  the  pecten 
equilateral. 

26  The  chief  impressions  left  on  the  shell  are  those  made  by  the  muscles 
—  the  dark  spots  called  "eyes"  by  oystermen  ;  the  pallial  line  made  by 


4/0  NOTES 

the  margin  of  the  mantle  ;   and  the  bend  in  the  pallia!  line,  called  pallial 
sinus,  which  exists  in  those  shells  having  retractile  siphons,  as  the  clam. 

27  The  clam  is  the  highest  of  lamellibranchs,  and  the  oyster  one  of  the 
lowest.     The  Mya  arenaria,  or  "  soft  clam,"  has  its  shell  always  open  a 
little;  while    Venus  mercenaria,  or  "hard  clam,"  keeps  its  shell  closed 
when  disturbed. 

28  The  slug  has  no  shell  to  speak  of.     It  may  be  remembered,  as  a  rule, 
that  all  univalve  shells  in  and  around  the  United  States  are  gastropods, 
and  that  all  bivalves  in  our  rivers  and  lakes,  and  along  our  seacoasts  (save 
a  few  brachiopods),  are  pelecypods  (lamellibranchs). 

29  Hold  the  shell  with  the  apex  up  and  the  mouth  toward  the  observer. 
If  the  mouth  is  on  his  right,  the  shell  is  right-handed  or  dextral,  if  on  his 
left,  sinistral.     In  other  words,  a  right  handed  shell  is  like  a  right-handed 
screw. 

30  Instead  of  a  strong  breathing  tube  with  a  valve,  answering  for  a  force- 
pump  and  propeller,  as  in  the  cuttlefish,  it  has  only  an  open  gutter  made 
by  a  fold  in  the  mantle,  like  the  siphons  of  the  gastropods.     The  back 
chambers  are  filled  with  gas. 

The  common  poulpe  has  two  thousand  suckers,  each  a  wonderful  little 
pump,  under  the  control  of  the  animal's  will. 

81  The  facial  angle  becomes  of  less  and  less  importance  as  we  go  away 
from  man,  and  for  two  reasons.  Where  the  brain  does  not  fill  the  brain 
case  the  angle  is  obviously  of  little  value,  and  if  the  jaws  are  largely  de- 
veloped the  angle  is  reduced,  although  intelligence  may  not  be  altered. 

32  Oblong  human  skulls,  whose  diameter  from  the  frontal  to  the  occipi- 
tal greatly  exceeds  the  transverse  diameter,  are  called  dolichocephalic ;  and 
such  are  usually  prognathous,  i.e.,  have  projecting   jaws,  as  the  negro's. 
Round  skulls,  whose  extreme  length  does  not  exceed  the  extreme  breadth 
by  a  greater  proportion  than  100  to  80,  are  brachycephalic ;  and  such  are 
generally  ortkognathous,  or  straight-jawed. 

33  The    classes   are  variously  grouped  into  the   Hematocrya,  or  cold- 
blooded, and  the  Hematotherma,  or  warm-blooded  ;   into  the  Branchiata 
and  Abranchiata  ;  into  the  Allantoidea  and  Anallantoidea. 

34  Amphibians  with  a  moist  skin  are  also  remarkable  for  their  cutaneous 
respiration.     They  will  live  many  days  after  the  lungs  are  removed.     Their 
vertebrae  vary  in  form :  in  the  lowest  they  are  biconcave,  like  those  of 
fishes  ;   in  salamanders  they  are  opisthocoelous  :   in  the  frogs  and  toads 
they  are  usually  proccelous. 

85  Salamanders  are  often  taken  for  lizards,  but  differ  in  having  gills  in 
early  life  and  a  naked  skin.     The  proteus  and  siren  resemble  a  tadpole 
arrested  in  its  development. 

86  The  Surinam  toad  has  no  tongue. 

37  There  are  some  notable  exceptions.  The  slow  worm  is  legless,  and 
the  chameleon  has  a  soft  skin,  with  minute  scales. 


NOTES 


471 


38  The  posterior  pair  of  limbs  is  sometimes  represented  by  a  pair  of 
small  bones;    and  the  boas   and  pythons  show  traces   of  external   hind 
limbs. 

39  The  plastron  is  formed  partly  of  dermal  and  partly  of  endoskeletal 
pieces. 

40  Knees  always  bend,  forward,  and  heels  always  bend  backward. 

41  We  cannot  claim  that  this  airy  skeleton  is  necessary  for  flight.     The 
bones  of  the  bat  are  free  from  air,  yet  it  is  able  to  keep  longer  on  the 
wing  than  the  sparrow.     The  common  fowl  has  a  hollow  humerus  ;  while 
some  birds  of  long  flight,  as  the  snipe  and  curlew,  have  airless  bones. 

42  Hopping  is  characteristic  of  and  confined  to  the  perchers  ;  but  many 
of  them,  as  the  meadow  lark,  blackbird,  and  crow,  walk. 

43  This  order  is  artificial.     But  it  is  better  to  retain  it  until  ornitholo- 
gists agree  upon  some  natural  arrangement. 

44  The  whales  are  hairy  during  foetal  life  only. 

45  The  manatee  has  6  ;    Hoffmann's  sloth  6  ;  and  two  species  of  three- 
toed  sloth  have  respectively  8  and  9. 

46  As  in  the  whale,  porpoise,  seal,  and  mole.     Teeth  are  wanting  in  the 
whalebone  whales,  ant-eaters,  manis,  and  echidna. 

47  The  monotremes  resemble  marsupials  in  having  marsupial  bones,  but 
have  no  pouch.     They  differ  from  all  other  mammals  in  having  no  distinct 
nipples. 

48  The  pouch  is  wanting  in  some  opossums  and  the  dasyurus. 

49  The  extinct  horse  {Hipparion)    had  three  toes,  two  small  hoofs  dan- 
gling behind.     The  foot  of  the  horse  is  of  wonderful  structure.     The  bones 
are  constructed  and  placed  with  a  view  to  speed,  lightness,  and  strength, 
and  bound  together  by  ligaments  of  marvelous  tenacity.     There  are  elastic 
pads  and  cartilages  to  prevent  jarring  ;   and  all  the  parts  are  covered  by  a 
living  membrane  which  is  exquisitely  sensitive,  and  endows  the  foot  with 
the  sense  of  touch,  without  which  the  animal  could  not  be  sure-footed. 
The  hoof  itself  is  made  of  parallel  fibers,  each  a  tube  composed  of  thousands 
of  minute  cells,  the  tubular  form  giving  strength.     There  are  three  parts, 
"wall,"  "sole,"  and  "frog" — the  triangular,  elastic  piece  in  the  middle, 
which  acts  as  a  cushion  to  prevent  concussion  and  also  slipping. 

60  The  fore  feet  of  the  tapir  have  four  toes,  but  one  does  not  touch  the 
ground. 

51  The  camel  and  llama  are  exceptional,  having  two  upper  incisors  and 
canines,  are  not  strictly  cloven-footed,  having  pads  rather  than  hoofs,  and 
are  hornless. 

52  For  the  best  account  of  the  elephant,  see  Tennant's  "  Ceylon  "  or 
Brehm's  "Thierleben." 

53  The  hyena  alone  of  the  carnivores  has  only  four  toes  on  all  the  limbs, 
and  the  dog  has  four  hind  toes. 

64  The  old  term  Quadrumana  is  rejected,  because  'it  misleads,  for  apes, 


472  NOTES 

as  well  as  men,  have  two  feet  and  two  hands.  There  is  as  much  anatomi- 
cal difference  between  the  feet  and  hands  of  an  ape  as  between  the  feet 
and  hands  of  man.  Owen,  however,  with  Cuvier,  considers  the  apes  truly 
"  four-handed." 

55  The  eye  orbits  of  the  lemurs  are  open  behind.      The  flying  lemur 
{Galeopithecus)  is  considered  an  insectivore, 

56  It  fails  to  cover  in  the  howling  monkey  and  siamang  gibbon  ;   but  in 
the  squirrel  monkey  it  more  than  covers,  overlapping  more  than  in  man. 
As  to  the  convolutions,  there  is  every  grade,  from  the  almost  smooth  brain 
of  the  marmoset  to  that  of  the  chimpanzee  or  orang,  which  falls  but  little 
below  man's. 

57  The  tailed  apes  of  the  Old  World  have  Iqnger  legs  than  arms,  and 
generally  have  "  cheek  pouches,"  which  serve  as  pockets  for  the  temporary 
stowage  of  food. 

58  In  the  human  infant,  the  sole  naturally  turns  inward;   and  the  arms 
of  the  embryo  are  longer  than  the  legs. 

59  The  aye-aye,  one  of  the  lowest  of  the  lemurs,  is  remarkable  for  the 
large  proportion  of  the  cranium  to  the  face. 

60  This  feature  was  shared  by  the  extinct  Anoplotherium,  and  now  to 
some  extent  by  one  of  the  lemurs  (  Tarsius). 

61  We  have  treated  man  zoologically  only.     His  place  in  nature  is  a 
wider  question  than  his  position  in  Zoology  ;   but  it  involves  metaphysical 
and  psychological  considerations  which  do  not  belong  here. 

62  This  twofold  division  is  arbitrary.     No  essential  distinction,  founded 
on  the  nature  of  the  elements  concerned,  or  the  laws  of  their  combination, 
can  be  made  ;    and  so  many  so-called  organic  substances,  as  urea,  am- 
monia, alcohol,  tartaric  and  oxalic  acids,  alizarine,  and  glucose,  have  been 
prepared  by  inorganic  methods,  that  the  boundary  line  is  daily  becoming 
fainter,  and  may  in  time  vanish  altogether.     We  would  here  utter  our  pro- 
test against  the  introduction  of  any  more  terms  like  inorganic,  invertebrate, 
acephalous,  etc.,  which  express  no  qualities. 

63  Even  the  works  of  nearly  all  animals,  as  nests  and  burrows,  are  bounded 
by  curved  lines. 

64  London  Quarterly  Review,  January,  1869,  p.  142.     It  is  true  of  any 
.great  primary  group  of  animals,  as  of  a  tree,  that  it  is  much  more  easy  to 
define  the  summit  than  the  base. 

65  "There  are  certain  phenomena,  even  among  the  higher  plants,  con- 
nected with  the  habits  of  climbing  plants  and  with  the  functions  of  fertiliza- 
tion, which  it  is  very  difficult  to  explain  without  admitting  some  low  form 
of  a  general  harmonizing  and  regulating  function,  comparable  to  such  an 
obscure  manifestation  of  reflex  nervous  action  as  we  have  in  sponges  and 
in  other  animals  in  which  a  distinct  nervous  system   is   absent."  —  Pro- 
fessor WYVILLE  THOMSON'S  Introductory  Lecture  at  Edinburgh. 

66  "  If  nature  had  endowed  us  with  microscopic  powers  of  vision,  and  the 


NOTES  473 

integuments  of  plants  had  been  rendered  perfectly  transparent  to  our  eyes, 
the  vegetable  world  would  present  a  very  different  aspect  from  the  apparent 
immobility  and  repose  in  which  it  is  now  manifested  to  our  senses."  — 
HUMBOLDT'S  Cosmos,  i.,  341. 

67  See  Gray's  "  Structural  Botany,"  6th  ed.,  Introduction  ;   also  Rolles- 
ton's  "  Forms  of  Animal  Life,"  Introduction. 

68  "  Life  has  been  called  the  vital  force,  and  it  has  been  suggested  that 
it  may  be  found  to  belong  to  the  same  category  as  the  convertible  forces, 
heat  and  light.     Life  seems,  however,  to  be  more  a  property  of  matter  in  a 
certain  state  of  combination  than  a  force.     It  does  no  work,  in  the  ordinary 
sense."  —  Professor  WYVILLE  THOMSON. 

69  The  vegetable  cell  usually  consists  of  a  cell  wall  surrounding  the  pri- 
mordial utricle  or  protoplasmic  sac.     In  animal  cells  the  former,  though 
often  present,  is  usually  not  easily  seen.     As  a  general  fact,  animal  cells 
are  smaller  than  vegetable  cells. 

70  Cells  are  not  the  sources  of  life,  as  once  thought,  but  are  the  products 
of  protoplasm.     "  They  are  no  more  the  producers  of  vital  phenomena  than 
the  shells  scattered  in  orderly  lines  along  the  sea  beach  are  the  instruments 
by  which  the  gravitation  force  of  the  moon  acts  upon  the  ocean.     Like 
these,  the  cells  mark  only  where  the  vital  tides  have  been  and  how  they 
have  acted."  —  Professor  HUXLEY. 

71  Many  of  the  bones  of  the  skull  are  preceded  by  membrane  —  hence 
called  membrane  bones, 

72  In  the  heart,  the  muscular  fibers  are  striated,  yet  involuntary  ;  but  the 
sarcolemma  is  wanting. 

73  We  may,  however,  infer  that  the  animal  functions  are  not  absolutely 
essential  to  the  vegetative,  from  the  facts  that  plants  digest  without  muscles 
or  nerves,  and  that  nutrition  takes  place  in  the  embryo  long  before  the 
nerves  have  been  developed. 

74  Scorpions  and  spiders  properly  feed  upon  the  juices  of  their  victims 
after  lacerating  them  with  their  jaws,  but  fragments  of  insects  have  been 
found  in  their  stomachs. 

75  The  real  tongue  forms  the  floor  of  the  mouth,  and  is  found  as  a  distinct 
part  in  a  few  insects,  as  the  crickets. 

76  In  the  cyclostomata,  it  is  circular  or  oval. 

77  The  mouth  of  the  whale  is  exceptional,  the  walls  not  being  dilatable. 
The  act  of  sucking  is  characteristic  of  all  young  mammals,  hence  the  need 
of  lips. 

78  The  ant-eater  has  two  callous  ridges  in  the  mouth,  against  which  the 
insects  are  crushed  by  the  action  of  the  tongue. 

79  The  baleen  plates  do  not  represent  teeth  ;   for  in  the  embryo  of  the 
whale  we  find  minute  calcareous  teeth  in  both  jaws,  which  never  cut  the  gum. 
The  whalebone  is  a  peculiar  development  of  hair  in  the  palate,  and  under 
the  microscope  it  is  seen  to  be  made  up  of  fibers  which  are  hollow  tubes. 


474  NOTES 

80  The  "  tusks  "  of  the  elephant  are  prolonged  incisors ;    those  of  the 
walrus,  wild  boar,  and  narwhal  are  canines. 

81  "  I  was  one  day  talking  with  Professor  Owen  in  the  Hunterian  Mu- 
seum, when  a  gentleman  approached,  with  a  request  to  be  informed  respect- 
ing the  nature  of  a  curious  fossil  which  had  been  dug  up  by  one  of  his 
workmen.     As  he  drew  the  fossil  from  a  small  bag,  and  was   about   to 
hand  it  for  examination,  Owen  quietly  remarked,  '  That  is  the  third  molar 
of  the  under  jaw  of  an  extinct  species  of  rhinoceros.' "  —  LEWES'S  Studies 
in  Animal  Life. 

82  This  gap  or  interspace,  so  characteristic  of  the  inferior  mammals,  is 
called  diastema.     It  is  wanting  in  the   extinct  anoplotherium,  is  hardly 
perceptible  in  one  of  the  lemurs,  and  is  not  found  in  man. 

83  In  the  spermaceti  whale,  the  teeth  are  fixed  to  the  gum. 

84  The  iguana  among  reptiles,  and  fishes  with  pavement  teeth,  approach 
the  mammal  in  this  respect. 

85  This  movement  is  called  peristaltic  or  vermicular,  and  characterizes 
all  the  successive  movements  of  the  alimentary  canal. 

86  Fishes  and  amphibians  have  no  saliva,  but  a  short  gullet.     Birds  are 
aided  by  a  sudden  upward  jerk  of  the  head. 

87  Fishes  and  reptiles  have  no  pharynx  proper,  the  nostrils  and  glottis 
opening  into  the  mouth. 

88  This  movement  of  the  pharynx  and  esophagus  is  wholly  involuntary. 
Liquids  are  swallowed  in  exactly  the  same  way  as  solids. 

89  The  few  animals  in  which  the  digestive  cavity  is  wanting  are  called 
agastric,  and  agree  in  having  a  very  simple  structure.     Such  are  some 
Entozoa  (as  tapeworm)  and  unicellular  Protozoa  (as  Gregarina}.     They 
absorb  the  juices,  already  prepared,  by  the  physical  process  of  endosmose. 
There  are  other  minute  organisms  (bacteria)  which  seem  to  be  able  to  ex- 
tract the  necessary  elements,  C  H  O  N,  from  the  medium  in  which  they  live. 

90  The   cavity  of  a  sponge  is  perhaps  homologous  with  the  digestive 
cavity,  but  is   not  functionally  such.     Each  cell  lining  it   does  its  own 
digestion,  taking  the  food  from  the  water  circulating  in  the  cavity. 

91  "  Nothing  is  more  curious  and  entertaining  than  to  watch  the  neat- 
ness and  accuracy  with  which  this  process  is  performed.     One  may  see  the 
rejected  bits  of  food  passing  rapidly  along  the  lines  upon  which  these 
pedicellariae  occur  in  greatest  number,  as  if  they  were  so  many  little  roads 
for  the  conveying  away  of  the  refuse  matters  ;   nor  do  the  forks  cease  from 
their  labor  till  the  surface  of  the  animal  is  completely  clean  and  free  from 
any  foreign  substance."  —  AGASSIZ'S  Seaside  Studies. 

92  In  the  larva  of  the  bee,  the  anal  orifice  is  wanting. 

93  The  length  of  the  canal  in  insects  is  not  so  indicative  of  the  habits  as 
in  mammals.     Thus,  in  the  carnivorous  beetle  the  canal  is  nearly  as  long 
as,  and  more  complicated  than,  it  is  in  the  nectar-sipping  butterflies. 

94  The  object  of  this  is  unknown.     It  does  not  occur  in  the  oyster. 


NOTES  475 

95  In  the  nautilus,  this  is  preceded  by  a  capacious  crop. 

96  In  the  shark,  this  is  impossible,  owing  to  a  great  number  of  fringes  in 
the  gullet  hanging  down  toward  the  stomach. 

97  At  the  beginning  of  the  large  intestine  in  the  lizards  (and  in  many 
vertebrates  above  them,  especially  the  vegetarian  orders),  there  is  a  blind 
sac,  called  cacmn. 

98  The  crocodile  is  said  to  swallow  stones  sometimes,  like  birds,  to  aid 
the  gastric  mill. 

99  In  the  crop  of  the  common  fowl,  vegetable  food  is  detained  sixteen 
hours,  or  twice  as  long  as  animal  food.     The  dormouse,  among  mammals, 
has  an  approach  to  a  crop. 

100  In  invertebrates,  the  gizzard,  when  present,  is  situated  between  the 
crop  and  the  true  stomach  ;   in  birds,  it  comes  after  the  stomach. 

101  The  tapeworm  has  no  digestive  apparatus,  but  absorbs  the  already 
digested  food  of  its  host.     This  is  no  exception  to  the  rule.     The  chemical 
preparation  of  the  food  has  preceded  its  absorption. 

102  \ye  finc[  tne  most  abundant  saliva  in  those  mammals  that  feed  on 
herbs  and  grain,  but  its  action  on  starch  is  extremely  feeble. 

103  The  acid  in  the  gastric  juice  has  an  important  function  in  killing  or 
preventing  the  growth  of  bacteria  which  are  taken  in  with  the  food.     The 
gastric  juice  also  dissolves  the  albuminous  walls  of  fat  cells,  thus  permitting 
the  contained  fats  to  escape.     The  drops  of  fat  fuse  together  into  larger 
masses,  which  are  later  broken  up  into  droplets  or  emulsified  by  the  pan- 
creatic juice. 

104  It  is  probablq  that  the  digestive  part  of  the  alimentary  canal  in  all 
animals  manifests  a  similar  mechanical  movement.     It  is  most  remarkable 
in  the  gizzard  of  a  fowl,  which  corresponds  to  the  pyloric  end  of  the  human 
stomach.     This  muscular  organ,  supplying  the  want  of  a  masticatory  appa- 
ratus in  the  head,  is  powerful  enough  to  pulverize  not  only  grain,  but  even 
pieces  of  glass  and  metal.     This  is  done  by  two  hard  muscles  moving 
obliquely  upon  each  other,  aided  by  gravel  purposely  swallowed  by  the 
bird.     The  grinding  may  be  heard  by  means  of  the  stethoscope. 

105  Chyle  is  opaque  in  carnivores  ;   more  or  less  transparent  in  all  other 
vertebrates,  as  in  birds,  since  the  food  does  not  contain  fatty  matter. 

106  In  fishes,  the  villi  are  few  or  wanting.     In  man,  they  number  about 
10,000  to  the  square  inch. 

107  Except,  perhaps,  the  tendons,  ligaments,  epidermis,  etc. 

108  The  blood  is  colorless  also  in  the  muscular  part  of  fishes.     That  of 
birds  is  of  the  deepest  red.     The  coloring  matter  of  the  red  blood  in  worms 
is  not  in  the  corpuscles,  but  in  the  plasma. 

109  Coagulation  may  be  artificially  arrested  for  a  brief  time  by  common 
salt.     Arterial  blood  coagulates  more  rapidly  than  venous.     The  disposi- 
tion of  the  red  corpuscles  in  chains,  or  rouleaux,  does  not  occur  within  the 
blood  vessels.     The  cause  has  not  been  discovered. 


476  NOTES 

110  The  corpuscles  of  invertebrates  are  usually  colorless,  even  when  the 
blood  is  tinged. 

111  Except  during  the  foetal  life.     The  corpuscles  of  the  camel  are  non- 
nucleated,  as  in  other  mammals.     If  the  transparent  fluid  from  a  boil  be 
examined  with  a  microscope,  it  will  be  seen  to  be  almost  entirely  composed 
of  colorless  corpuscles. 

112  There  are  no  valves  in  the  veins  of  fishes,  reptiles,  and  whales,  and 
few  in  birds. 

113  Capillaries  are  wanting  in  the  epidermis,  nails,  hair,  teeth,  and  carti- 
lages.    Hence,  the  epidermis,  for  example,  when  worn  out  by  use,  is  not 
removed  by  the  blood,  like  other  tissues,  but  is  shed. 

114  A  part  of  the  blood,  however,  in  going  from  the  capillaries  of  the 
digestive  organs  to  the  heart,  is  turned  aside  and  made  to  pass  through  the 
liver  and  kidneys  for  purification.     This  is  called  the  portal  circulation, 
and  exists  in  all  vertebrates,  except  that  in  birds  and  mammals  it  is  con- 
fined to  the  liver. 

115  Two  in  the  higher  mammals,  three  in  the  lower  mammals,  birds,  and 
reptiles.     They  are  called  vena  cava. 

116  Tricuspid  in  mammals,  triangular  in  birds. 

117  The  pulse  of  a  hen  is  140  ;  of  a  cat,  1 10  to  120  ;  of  a  dog,  90  to  100  ; 
and  of  an  ox,  25  to  42. 

118  The  bivalve  brachiopods  breathe  by  delicate  fringed  arms  about  the 
mouth,  and  by  the  "  mantle. " 

119  The  air  bladder,  found  in  most  fishes,  is  another  rudiment  of  a  lung, 
although  it  is  used,  not  for  respiration,  but  for  altering  the  specific  gravity 
of  the  fish.     In  the  gar  pike  of  our  Northern  lakes  it  very  closely  resem- 
bles a  lung,  having  a  cellular  structure,  a  tracheal  tube,  and  a  glottis.     It  is 
here  functional.    The  gills  represent  lungs  only  in  function  ;  they  are  totally 
distinct  parts  of  the  organism. 

12)  In  the  human  lungs  they  number  600,000,000,  each  about  TJ7  of  an 
inch  in  diameter,  with  an  aggregate  area  of  132  square  feet.  The  thickness 
of  the  membrane  between  the  blood  and  the  air  is  ^Q^  of  an  inch.  The 
lungs  of  carnivores  are  more  highly  developed  than  those  of  herbivores. 
In  the  manatee,  they  are  not  confined  to  the  thorax,  but  extend  down 
nearly  to  the  tail. 

121  Crocodiles  are  the  only  reptiles  whose  nostrils  open  in  the  throat  be- 
hind the  palate,  instead  of  directly  into  the  mouth  cavity.     This  enables 
the  crocodile  to  drown  its  victim  without  drowning  itself ;   for,  by  keeping 
its  snout  above  water,  it  can  breathe  while  its  mouth  is  wide  open. 

122  A  rudimentary  diaphragm  is  seen  in  the  crocodile  and  ostrich. 

123  The  poison  glands  of  venomous  serpents  and  the  silk  vessels  of  cater- 
pillars are  considered  to  be  modified  salivary  glands.     Birds,  snakes,  and 
cartilaginous  fishes  have  no  urinary  bladder. 

124  Since  the  weight  of  a  full-grown  animal  remains  nearly  uniform,  it 


NOTES  477 

must  lose  as  much  as  it  receives  ;   that  is,  the  excretions,  including  the 
solid  residuum  ejected  from  the  intestinal  canal,  equal  the  food  and  drink. 

125  Other  names  for  derm  are,  cutis,  corium,  enderon,  and  true  skin ; 
and  for  epidermis,  cuticle,  ecderon,  and  scarfskin.     The  derm  is  often  so 
intimately  blended  with  the  muscles  that  its  existence  as  a  distinct  layer  is 
not  easily  made  out. 

126  Papillae  are  scarcely  visible  in  the  skin  of  reptiles  and  birds. 

127  The  animal  basis  of  this  structure  is  chitin,  a  peculiar  hornlike  sub- 
stance found  in  the  hard  parts  of  all  the  articulated  animals. 

128  The  shell  is  always  an  epidermal  structure,  even  when   apparently 
internal.     The  horny  "pen"  of  the  squid,  the  "bone"  of  the  cuttlefish, 
and  the  calcareous  spot  on  the  back  of'the  slug  are  only  concealed  under 
a  fold  of  the  mantle.     So  the  shell  of  the  common  unio,  or  fresh-water 
clam,  is  covered  with  a  brownish  or  greenish  membrane,  which  is  the 
outer  layer  of  the  epidermis.     Where  the  mantle  covers  the  lips  of  a  shell, 
as  in  most  of  the  large  sea  snails,  or  where  its  folds  cover  the  whole  ex- 
terior, as  in  the  polished  cowry,  the  epidermis  is  wanting,  or  covered  up 
by  an  additional  layer. 

129  The  pearls  of  commerce,  found  in  the  mantle  of  some  mollusks,  are 
similar  in  structure  to  the  shell  ;   but  what  is  the  innermost  layer  in  the 
shell  is  placed  on  the  outside  in  the  pearl,  and  is  much  finer  and  more 
compact.     The  pearl  is  formed  around  some  nucleus,  as  an  organic  particle, 
or  grain  of  sand. 

130  \vhen  the  centrum  is  concave  on  both  sides,  as  in  fishes,  it  is  said  to 
be  amphiccelous ;  when  concave  in  front  and  convex  behind,  as  in  croco- 
diles, it  is  called  proccelous  ;  when  concave  behind  and  convex  in  front,  as 
in  the  neck-vertebrae  of  the  ox,  it  is  opisthoccelous.     In  the  last  two  cases, 
the  vertebras  unite  by  ball-and-socket  joints. 

131  Whether  the  skull  represents  any  definite  number  of  vertebrae  was  long 
under  discussion.     We  cannot  speak  of  "  cranial  vertebrae  "  in  the  same 
sense  as  "  cervical  vertebrae."     The  most  that  can  be  said  is  that  in  a  gen- 
eral way  the  skull  is  homologous  to  part  of  the  vertebral  column. 

132  A  few  have  but  one  pair,  the  whale  and  siren  wanting  the  hind  pair  ; 
while  some  have  none  at  all,  as  the  snakes  and  lowest  vertebrates.     In 
land  animals,  the  posterior  limbs  are  generally  most  developed  ;   in  aquatic 
animals,  the  anterior.     Dr.  Wyman  contends  that  the  limbs  are  tegumen- 
tary  organs,  and  attached  to  the  vertebral  column  in  the  same  sense  that 
the  teeth  are  attached  to  the  jaws.     Other  theories  are  that  they  originate 
from  gill  arches  (Gegenbaur)  or  that  they  are  remains  of  a  once  continu- 
ous lateral  fin  (Thacher). 

133  The  first  trace  of  muscular  tissue  is  found'  in  the  stem  of  vorticella  — 
an  infusorian.     In  hydra  we  find  neuro-muscular  cells,  and  the  jellyfishes 
have  muscular  tissue. 

134  The  muscles  of  some  invertebrates,  as  spiders,  are  yellow. 


478  NOTES 

185  The  muscles  of  the  heart  and  gullet  are  striped.  In  the  lower  ani- 
mals these  distinctions  of  voluntary  and  involuntary,  striated  and  smooth, 
solid  and  hollow,  muscles  can  seldom  be  made. 

136  The  skeleton  of  the  carrion  crow,  for  example,  weighs,  when  dry, 
only  twenty-three  grains. 

137  The  dragon  fly  can  outstrip  the  swallow  ;   nay,  it  can  do  in  the  air 
more  than  any  bird  —  it  can  fly  backward  and  sidelong,  to  right  or  left,  as 
well  as  forward,  and  alter  its  course  on  the  instant  without  turning.     It 
makes  twenty-eight  beats  per  second  with  its  wings,  while  the  bee  makes 
one  hundred  and  ninety,  and  the  house  fly  three  hundred  and  thirty.     The 
swiftest  race  horse  can  run  at  double  the  rate  of  the  salmon.     So  that 
insect,  bird,  quadruped,  and  fish,  would  be  the  order  according  to  velocity 
of  movement. 

138  The  theory  that  flies  adhere  by  atmospheric  pressure  is  now  aban- 
doned. 

189  More  precisely,  the  term  brain  applies  only  to  the  cerebrum,  while 
the  total  contents  of  the  cranium  are  called  encephalon. 

140  The  exact  functions  of  the  cerebrum  are  not  yet  clearly  understood. 
If  we  remove  it  from  fishes,  or  even  birds,  their  voluntary  movements  are 
little  affected,  while  the  Amphioxus,  the  lowest  of  fishes,  has  no  brain  at 
all,  but  its  life  is  regulated  by  the  spinal  cord.     Such  mutilated  animals, 
however,  make  no  intelligent  efforts.     The  substance  of  the  cerebrum,  as 
also  the  cerebellum,  is  insensible,  and  may  be  cut  away  without  pain  to 
the  animal;   and  when  both  are  thus  removed,  the  animal  still  retains 
sensation,  but  not  consciousness. 

141  It  is  very  difficult  to  define  sensation,  or  sensibility.     The  power  is 
possessed  by  animals  which  have  neither  nervous  system  nor  consciousness. 
These  low  manifestations  of  sensibility  are  called  irritability  —  the  power- 
by  which  an  animal  is  capable  of  definitely  responding  to  a  stimulus  from 
without.     The  response  is  not  called  out  by  the  direct  action  of  the  stimu- 
lus, but  is  determined  mainly  by  the  internal  structure  and  condition  of 
the  animal. 

142  Parts  destitute  of  blood  vessels,  as  hair,  teeth,  nails,  cartilage,  etc., 
are  not  sensitive. 

143  «  Tentacles  "  and  "  horns  "  are  more  or  less  retractile,  while  antennae 
are  not,  but  are  hollow.     Antennae  alone  are  jointed. 

144  In  man,  the  soft  palate  and  tonsils  also  have  the  power  of  tasting. 

145  No  organ  of  hearing   has   been    discovered  with    certainty   in   the 
radiates  and  spiders.     The  "  ear  "  of  many  lower  animals  is  probably  an 
organ  for  perceiving  the  animal's  position  rather  than  sound  —  an  "  equi- 
librium organ." 

146  It  is  wanting  in  the  aquatic  mammals.      Crocodiles  have   the  first 
representative  of  an  outside  ear  in  the  form  of  two  folds  of  skin. 

147  This,  like  the  definition  of  smell  and  hearing,  is  loose  language. 


NOTES  479 

There  is  no  such  thing  as  sound  till  the  vibrations  strike  the  tympanum, 
nor  even  then,  for  it  is  the  work  of  the  brain,  not  of  the  auditory  nerve. 
Sound  is  the  sensation  produced  by  the  wave  movement  of  the  air.  If  thus 
defined  in  terms  of  sensation,  light  is  nothing  ;  without  eyes  the  world 
would  be  wrapped  in  darkness.  Some  Protozoa,  as  Euglena,  have  a  pigment 
spot  as  an  eye. 

148  In  invertebrates  and  aquatic  vertebrates,  the  crystalline  lens  is  globu- 
lar; or,  in  other  words,  it  is  round  in  short-sighted  animals,  and  flattish  in 
the  long-sighted.     The  lens  of  the  invertebrate  is  not  exactly  the  same  as 
the  lens  of  the  vertebrate  eye,  though  it  performs  the  same  function;   it  is 
really  a  part  of  the  cornea. 

149  The  ant  has  50  in  each  eye,  the  house  fly  400x5,  the  dragon  fly  28,000. 

150  The  pigment,  therefore,  while  apparently  in  front  of  the  retina,  is 
really  behind  it,  as  in  vertebrates.     The  layer  beneath  the  cornea,  serving 
as  an  "  iris,"  is  wanting  in  nocturnal  insects,  since  they  need  every  ray  of 
light.     The  optic  nerve  alone  is  insensible  to  the  strongest  light. 

151  It  should  be  noticed  that  this  corresponds  with  another  peculiar  fact 
already  mentioned,  that  either  hemisphere  of  the  brain  controls  the  muscles 
on  the  opposite  side  of  the  body.     In  invertebrates,  the  motor  apparatus  is 
governed  on  its  own  side. 

152  Sharks  have  eyelids,  while  snakes  have  none.     The  third  eyelid  (called 
nictitating  membrane}  is  rudimentary  in  many  mammals.     It  may  be  seen 
at  the  inner  angle  of  the  eye. 

153  An  infant  would  doubtless  learn  to  walk  if  brought  up  by  a  wild 
beast,  since  it  was  made  to  walk,  just  as  an  Infusorium  moves  its  cilia,  not 
because  it  has  any  object,  but  because  it  can  move  them.      Newborn 
puppies,  deprived  of  brains,  have  suckled;  and  decapitated  centipedes  run 
rapidly.     Such  physical  instincts  exist  without  mind,  and  may  be  termed 
"  blind  impulses." 

154  \Ye  say  "  apparently,"  because  it  may  be  a  fixed  habit,  first  learned 
by  experience,  transmitted  from  generation  to  generation.    A  duckling  may 
go  to  the  water,  and  a  hound  may  follow  game  in  some  sense,  as  Sir  John 
Herschel  devoted  himself  to  astronomy,  inheriting  a  taste  from  his  father. 
Breeders  take  advantage  of  this  power  of  inheritance. 

155  \ye  may  divide  the  apparently  voluntary  actions  of  animals  into  three 
classes.     First,  organic,  in  which  consciousness  plays  no  part,  and  which  are 
due  wholly  to  the  animal  machine.     Second,  instinctive,  in  which  conscious- 
ness may  be  present,  but  which  are  not  controlled  by  intelligence.     Third, 
associative,  in  which  the  animals  act  under  conscious  combination  of  dis- 
tinct, single  ideas,  or  past  impressions.     To  these  we  may  add  rational 'acts, 
in  which  the  mental  process  takes  place  under  the  laws  of  thought. 

156  "Thus,  while  the  human  organism  may  be  likened  to  a  keyed  instru- 
ment, from  which  any  music  it  is  capable  of  producing  can  be  called  forth 
at  the  will  of  the  performer,  we  may  compare  a  bee,  or  any  other  insect,  to 


480  NOTES 

a  barrel  organ,  which  plays  with  the  greatest  exactness  a  certain  number  of 
tunes  that  are  set  upon  it,  but  can  do  nothing  else."  — .CARPENTER'S  Mental 
Physiology,  p.  61.  This  constancy  may  be  largely  due  to  the  uniformity 
of  conditions  under  which  insects  live. 

157  We  may  say,  as  a  rule,  that  the  proportion  of  instinct  and  intelligence 
in  an  animal  corresponds  to  the  relative  development  of  the  spinal  cord 
and  cerebrum.     As  a  rule,  also,  the  addition  of  the  power  to  reason  comes 
in  with  the  addition  of  a  cerebrum,  and  is  proportioned  to  its  development. 
Between  the  lowest  vertebrate  and  man,  therefore,  we  observe  successive 
types  of  intelligence.     Intelligence,  however,  is  not  according  to  the  size 
of  the  brain  (else  whales  and  elephants  would  be  wisest),  but  rather  to  the 
amount  of  gray  matter  in  it.     A  honeycomb  and  an  oriole's  nest  are  con- 
structed with  more  care  and  art  than  the  hut  of  the  savage.     It  is  true, 
this  is  no  test  of  the  capability  of  the  animal  in  any  other  direction  ;  but 
when  they  are  fashioned  to  suit  circumstances,  there  is  proof  of  intelligence 
in  one  direction. 

158  An  exception.to  the  general  rule  that  the  smaller  animals  have  more 
acute  voices. 

159  It  is  wanting  in  a  few,  as  the  storks. 

160  The  nightingale  and  crow  have  vocal  organs  similarly  constructed, 
yet  one  sings  and  the  other  croaks. 

lei  Egg  cells  and  sperm  cells  are  detached  portions  of  the  parental  or- 
ganisms. Generally,  these  two  kinds  of  cells  are  produced  by  separate 
sexes;  but  in  some  cases,  as  the  snail,  they  originate  in  the  same  individual. 
Such  an  animal,  in  which  the  two  sexes  are  combined,  is  called  an  her- 
maphrodite 

162  The  eggs  of  mammals  are  of  nearly  uniform  size;   those  of  birds, 
insects,  and  most  other  animals  are  proportioned  to  the  size  and  habits  of 
the  adult.     Thus,  the  egg  of  the  gepyornis,  the  great  extinct  bird' of  Mada- 
gascar, has  the  capacity  of  fifty  thousand  humming-birds'  eggs. 

163  As  a  general  rule,  when  both  sexes  are  of  gay   and   conspicuous 
colors,  the  nest  is  such  as  to  conceal  the  sitting  bird;  while,  whenever 
there  is  a  striking  contrast  of  colors,  the  male  being  gay  and  the  female 
dull,  the  nest  is  open.     Such  as  form  no  nest  are  many  of  the  waders, 
swimmers,  scratchers,  and  goatsuckers. 

164  This  lies  at  first  transversely  to  the  long  axis  of  the  egg.     As  the 
chick  develops,  it  turns  upon  its  side. 

165  The  blood  appears  before  the  true  blood  vessels,  in    intercellular 
spaces.     It  is  at  first  colorless,  or  yellowish. 

166  Exactly  as  the  blood  in  the  capillaries  of  the  lungs  is  aerated  by  the 
external  air. 

167  Thus,  the  hollow  wing  bone  was  at  first  solid,  then  a  marrow  bone, 
and  finally  a  thin-walled  pneumatic  bone.     The  solid  bones  of  penguins 
are  examples  of  arrested  development. 


NOTES  481 

168  The  thigh  bone  ossifies  from   five   centers.     The    bone    eventually 
unites  to  one  piece. 

169  jror   this   reason,    mammals   are    called    viviparous ;   but,    strictly 
speaking,  they  are  as  oviparous  as  birds.     The  process  of  reproduction  is 
the  same,  whether  the  egg  is  hatched  within  the  parent  or  without.     The 
eggs  of  birds  contain  whatever  is  wanted  for  the  development  of  the  em- 
bryo, except  heat,  which  must  come  from  without.     Mammals,  having  no 
food  yolk,  obtain  their  nutrition  from  the  blood  of  the  parent,  and  after 
birth  from  milk. 

170  The  larvae  of  butterflies  and  moths  are  called  caterpillars  ;  those  of 
beetles,  grubs ;    those    of   flies,  maggots ;    those    of   mosquitoes,  -wigglers. 
The  terms  larva,  pupa,  and  imago  are  relative  only  ;    for,  while  the  grub 
and  caterpillar  are  quite  different  from  the  pupa,  the  bee  state  is  reached 
by  a  very  gradual  change  of  form,  so  that  it  is  difficult  to  say  where  the 
pupa  ends  and  the  imago  begins.     In  fact,  a  large  number  of  insects  reach 
maturity  through  an  indefinite  number  of  slight  changes.     The  bumblebee 
moults  at  least  ten  times  before  arriving  at  the  winged  state. 

171  Every  tissue  of  the  larva  disappears  before  the  development  of  the 
new  tissues  of  the  imago  is  commenced.     The  organs  do  not  change  from 
one  into  the  other,  but  the  new  set  is  developed  out  of  formless  matter. 
The  pupa  of  the  moth  is  protected  by  a  silken  cocoon,  the  spinning  of 
which  was  the  last  act  of  the  larva  ;   that  of  the  butterfly  is  simply  inclosed 
in  the  dried  skin  of  the  larva,  which  is  called  chrysalis  because  of  the 
golden  spots  with  which  it  is  sometimes  marked.     The  pupa  of  the  honey- 
bee is  called  nymph ;  it  is  kept  in  a  wax  cell  lined  with  silk,  which  the 
larva  spins.     The  time  required  to  pass  from  the  egg  to  the  imago  varies 
greatly  ;   the  bee  consumes  less  than  twenty  days,  while  the  cicada  requires 
seventeen  years. 

172  Compare  the  amount  of  food  required  in  proportion  to  the  bulk  of 
the  body,  and  also  with  the  amount  of  work  done,  in  youth,  manhood,  and 
old  age. 

173  Excepting,  perhaps,  that  the  new  tail  of  a  lizard  is  cartilaginous. 

174  The  patella,  or  kneepan,  has  no  representative  in  the  adult  fore 
limb. 

no  «  The  structure  of  the  highest  plants  is  more  complex  than  is  that  of 
the  lowest  animals ;  but,  for  all  that,  powers  are  possessed  by  jellyfishes  of 
which  oaks  and  cedars  are  devoid."  —  MIVART. 

176  It  is,  however,  true  that  the  number  of  eggs  laid  is  proportioned  to 
the  risk  in  development. 

177  See  Lewes's  charming  "  Studies  of  Animal  Life."  Doubtless  an 
examination  of  all  the  strata  of  the  earth's  crust  would  disclose  forms 
immensely  outnumbering  all  those  at  present  known.  And  even  had  we 
every  fossil,  we  should  have  but  a  fraction  of  the  whole,  for  many  deposits 
have  been  so  altered  by  heat  that  all  traces  have  been  wiped  out.  Animal 
DODGE'S  GEN.  ZOOL.  —  3  i 


482  NOTES 

life  is  much  more  diversified  now  than  it  was  in  the  old  geologic  ages ;   for 
several  new  types  have  come  into  existence,  and  few  have  dropped  out. 

178  Among  the  types  characteristic  of  America  are  the  gar  pike,  snapping 
turtle,  hummers,  sloths,  and  muskrat.     Many  of  our  most  common  animals 
are  importations  from  the  Old  World,  and  therefore  are  not  reckoned  with 
the  American  fauna;   such  as  the  horse,  ox,  dog,  and  sheep,  rats  and  mice, 
honeybee,  house  fly,  weevil,  currant  worm,  meal  worm,  cheese  maggot, 
cockroach,  croton  bug,  carpet  moth,  and  fur  moth.     Distribution  is  com- 
plicated by  the  voluntary  migration  of  some  animals,  as  well  as  by  man's 
intervention.     Besides  birds,  the  bison  and  seal,  some  rats,  certain  fishes, 
as  salmon  and  herring,  and  locusts  and  dragonflies    among   insects,  are 
migratory. 

179  when  the  cable  between  France  and  Algiers  was  taken  up  from  a 
depth  of  eighteen  hundred  fathoms,  there  came  with  it  an  oyster,  cockle 
shells,  annelid  tubes,  polyzoa,  and  sea  fans.     Ooze  brought  up  from  the 
Atlantic  plateau  (two  thousand  fathoms)  consisted  of  ninety-seven  per 
cent  of  foraminifers. 


THE    NATURALIST'S   LIBRARY 


THE  following  works  of  reference,  accessible  to  the  American  student, 
are  recommended :  — 


General  Works  and  Text-books 

PARKER  and  HASWELL,  Text-book  of  Zool- 
ogy- 

CAMBRIDGE  Natural  History. 

KINGSLEY,  Elements  of  Comparative  Zool- 
ogy. 

DAVENPORT,  Introduction  to  Zoology. 

JORDAN  and  KELLOGG,  Animals. 

AGASSIZ,  Methods  of  Study  in  Natural  His- 
tory. 

AGASSIZ  and  GOULD,  Principles  of  Zoology. 

ROLLESTON,  Forms  of  Animal  Life. 

LEWES,  Studies  of  Animal  Life. 

HUXLEY  and  MARTIN,  Elementary  Practi- 
cal Biology. 

OWEN,  Comparative  Anatomy  of  Inverte- 
brates and  Vertebrates. 

PARKER  and  PARKER,  Practical  Zoology. 

MORSE,  First  Book  of  Zoology. 

PACKARD,  Zoology. 

GEGENBAUR,  Comparative  Anatomy. 

PARKER,  Zootomy. 

PARKER,  Elementary  Biology. 

KINGSLEY,  The  Riverside  Natural  History. 

THOMSON,  Outlines  of  Zoology. 

CLAUS  and  SEDGWICK,  Text-book  of  Zool- 
ogy. 

THOMSON,  The  Study  of  Animal  Life. 

LANKESTER,  Zoological  Articles. 

MARSHALL  and  HURST,  Junior  Course  in 
Practical  Zoology. 

LANG,  Comparative  Anatomy. 

SCHMEIL,  Introduction  to  Zoology. 

Invertebrates 

HUXLEY,  Anatomy  of  Invertebrated  Ani- 
mals. 

MACALLISTER,  Introduction  to  Animal 
Morphology. 

BROOKS,  Handbook  of  Invertebrate  Zool- 
ogy. 

SIEBOLD,  Anatomy  of  Invertebrates. 

SHIPLEY,  Zoology  of  the  Invertebrata. 

McMuRRiCH,  Invertebrate  Zoology. 

PACKARD,  Text-book  of  Entomology. 


Vertebrates 

HUXLEY,  Anatomy  of  Vertebrated  Animals. 

HUXLEY  and  HAWKINS,  Atlas  of  Compara- 
tive Osteology. 

FLOWER,  Osteology  of  Mammalia. 

CHAUVEAU,  Comparative  Anatomy  of  Do- 
mesticated Animals. 

MIVART,  Lessons  in  Elementary  Anatomy. 

WIEDERSHEIM,  Comparative  Anatomy  of 
Vertebrates. 

MIVART,  The  Cat. 

GRAY,  Anatomy,  Descriptive  and  Surgical. 

QUAIN,  Human  Anatomy. 

REIGHARD  and  JENNINGS,  The  Cat. 

Embryology 

BALFOUR,  Comparative  Embryology. 

FOSTER  and  BALFOUR,  Elements  of  Em- 
bryology. 

PACKARD,  Life  Histories  of  Animals. 

MINOT,  Human  Embryology. 

MARSHALL,  Vertebrate  Embryology. 

HERTWIG,  Text-book  of  Embryology :  Man 
and  Mammals. 

KORSCHELT  and  HEIDER,  Invertebrate  Em- 
bryology. 

Physiology 

HUXLEY,  Lessons  in  Elementary  Physiol- 
ogy. 

CARPENTER,  Comparative  Physiology. 

FOSTER,  Text-book  of  Physiology. 

MARTIN,  The  Human  Body. 

GRIFFITHS,  Physiology  of  the  Inverte- 
brates. 

LANDOIS  and  STIRLING,  Human  Physi- 
ology. 

BINET,  Psychic  Life  of  Micro-organisms. 

MORGAN,  Animal  Life  and  Intelligence. 

Geographical  Distribution 
WALLACE,    Geographical    Distribution  of 

Animals. 
MURRAY,     Geographical     Distribution     of 

Mammals. 
BEDDARD,  Zoogeography. 


483 


THE   NATURALIST'S    LIBRARY 


Microscopy 

CARPENTER,  The  Microscope  and  its  Reve- 
lations. 

GRIFFITHS  and  HENFREY,  The  Micro- 
graphic  Dictionary. 

Evolution 

SCHMIDT,  Descent  and  Darwinism. 
HAECKEL,  History  of  Creation. 
DARWIN,  Origin  of  Species. 
HUXLEY,  Lay  Sermons,  etc. 
MIVART,  Lessons  from  Nature. 
ROMANES,   Darwin  and   after   Darwin:  I. 

The  Darwinian  Theory. 
ROMANES,     The    Scientific    Evidences    of 

Organic  Evolution. 
MARSHALL,    Lectures    on    the    Darwinian 

Theory. 
WEISMANN,  Essays  on  Heredity. 

Special  Works 

CLARK,  Mind  in  Nature. 

AGASSIZ,  Seaside  Studies  in  Natural  His- 
tory. 

TAYLOR,  Half-hours  at  the  Seaside. 

KENT,  Manual  of  the  Infusoria. 

GREENE,  Manuals  of  Sponges  and  Ccelen- 
terata. 

DANA,  Corals  and  Coral  Islands. 

DARWIN,  Vegetable  Mould  and  Earth- 
worms. 


VERRILL  and  SMITH,  Invertebrates  of  Vine- 
yard Sound. 

GOULD  and  BINNEY,  Invertebrata  of  Mas- 
sachusetts. 

WOODWARD,  Manual  of  Mollusca. 

HYATT,  Insecta. 

PACKARD,  Guide  to  the  Study  of  Insects. 

COMSTOCK,  Manual  for  the  Study  of  Insects. 

HOLLAND,  The  Butterfly  Book. 

HOWARD,  The  Insect  Book. 

SMITH,  Economic  Entomology. 

DUNCAN,  Transformation  of  Insects. 

JORDAN,  Manual  of  the  Vertebrates  of  the 
Northern  United  States. 

COUES,  Key  to  North  American  Birds. 

CHAPMAN,  Handbook  of  Birds  of  Eastern 
North  America. 

BAIRD,  BREWER,  and  RIDGWAY,  Birds  of 
North  America. 

BAIRD,  Mammals  of  North  America. 

ALLEN,  Mammalia  of  Massachusetts. 

FLOWER  and  LYDEKKER,  Mammals,  Living 
and  Extinct. 

SCAMMON,  Marine  Mammals  of  North 
Pacific. 

HARTMANN,  Anthropoid  Apes. 

PESCHEL,  The  Races  of  Man. 

MARSH,  Man  and  Nature. 

TYLOR,  Primitive  Culture. 

NICHOLSON,  Palaeontology. 

POULTON,  The'Colors  of  Animals. 


Of  serial  publications,  the  student  should  have  access  to  the  American 
Naturalist,  Science,  American  Journal  of  Science,  Popular  Science 
Monthly,  Smithsonian  Contributions,  and  Miscellaneous  Collections,  Bul- 
letins and  Proceedings  of  the  various  societies,  Annals  and  Magazine  of 
Natural  History,  and  Nature, 

The  following  works  are  recommended  as  having  no  English  equiva- 
lents :  — 


VOGT  ET   VUNG,   Traite   d'anatomie  com- 
pare'e  pratique. 

Also  the  periodicals :  — 

Zoologischer  Anzeiger. 


BRONN,  Classen  und  Ordnungen  des  Thier- 
reichs  (unfinished  and  expensive,  but 
indispensable  to  the  working  zoologist). 


Biologisches  Centralblatt. 


APPENDIX 


THE  following  directions  for  experiments  are  given  for  the 
purpose  of  enabling  the  teacher  and  pupil  to  make  further 
direct  observation  of  the  structure  and  functions  of  animals,  and 
are  supplementary  to  those  given  under  the  head  of  "  Practical 
Zoology." 

The  experiments  and  dissections  are  purposely  chosen* with 
a  view  to  their  simplicity,  and  to  the  ease  with  which  they  may 
be  performed.  Constant  reference  is  made  to  figures  which 
will  both  guide  and  illustrate  the  dissections.  More  extended 
studies  may  be  carried  out  with  the  aid  of  the  various  works 
mentioned  on  pages  483,  484. 

CHAPTER  V 

The  difficulty  of  distinguishing  by  ocular  observation  alone 
the  lower  animals  from  the  lower  plants  may  be  illustrated  by 
making  a  microscopic  examination  of  drops  of  sediment  from 
the  bottom  of  a  stagnant  ditch.  The  water  will  probably  be 
teeming  with  unicellular  organisms,  both  animal  and  vegetable, 
which  cannot  be  differentiated  by  characters  of  form,  size,  color, 
motion,  etc.,  alone. 

CHAPTER  VII 

It  is  especially  important  that  the  student  become  as  familiar 
as  possible  with  protoplasm  by  a  personal  study  of  its  structure 
and  physiology.  For  this  purpose  the  most  favorable  objects 
are  the  Protozoa,  which  are  readily  obtained  and  easily  pre- 
pared for  examination.  Directions  are  given  on  page  23. 
Compare  with  these  the  protoplasm  seen  in  the  cells  of  the 

485 


486  APPENDIX 

water  plants,  as  Nitella,  Chara  (end  cells  of  leaves,  and  in  the 
colorless  rhizoids),  and  Anacharis;  in  the  stamen  hairs  of 
Tradescantia ;  in  Spirogyra ;  in  the  cells  of  the  bulb  scales  of 
the  onion,  etc. 

CHAFFER   VIII 

In  studying  protoplasm,  many  kinds  of  cell  will  probably  be 
seen.  Those  mentioned  are  especially  large,  and  in  them  the 
protoplasm  is  likely  to  be  in  quite  active  motion.  To  illustrate 
cell  structure  use  not  only  the  lowest  organisms,  but  also  iso- 
lated cells  from  higher  animals  and  plants  —  for  example,  blood 
cells  from  the  frog  and  from  the  human  body.  Frog's  blood 
may  be  obtained  by  killing  the  animal  in  a  box  in  which  has 
been  placed  a  small  wad  of  cotton  saturated  with  chloroform; 
as  soon  as  the  frog  is  dead  cut  into  its  skin  to  make  the  blood 
flow,  then  on  a  glass  slide  mix  a  drop  of  the  blood  with  a  drop 
of  a  .75  per  cent  solution  of  salt  in  water,  put  on  a  cover  glass, 
and  examine  under  a  one- fourth  to  one-sixth  inch  objective 
(Figs.  260,  261).  Human  blood  maybe  obtained  by  pricking  the 
finger  and  mounting  the  drop  in  the  same  manner  (Fig.  259). 
Study  also  the  cells  seen  in  a  drop  of  saliva.  Some  of  these, 
the  salivary  corpuscles,  are  small  and  usually  spherical  in  shape  ; 
others,  the  epithelium  cells,  come  mainly  from  the  lining  mem- 
brane of  the  mouth,  are  polygonal  in  outline,  have  a  large  nu- 
cleus, and  are  frequently  found  in  groups  consisting  of  several 
cells.  Ciliated  cells  are  easily  obtained  by  placing  in  a  drop  of 
water  on  a  slide  a  small  portion  of  the  gill  of  a  live  oyster  or 
clam,  and  picking  it  to  pieces  with  dissecting  needles  (ordinary 
cambric  needles  fixed  by  the  eye  end  into  wooden  penholders) . 
Examine  under  a  one-fourth  or  one-fifth  inch  objective.  Some 
of  the  pieces  will  probably  be  seen  swimming  about  by  means  of 
their  cilia  (Fig.  199,  £).  With  these  animal  cells  compare  such 
vegetable  cells  as  pollen  grains,  spores  of  fungi,  the  cells  com- 
posing the  bodies  of  some  of  the  fresh-water  algae,  etc. 

As  the  satisfactory  preparation  of  the  tissues  requires  skill 
obtained  only  by  long  training  in  manipulation  and  in  the  use 
of  hardening  fluids,  stains,  etc.,  in  many  cases  it  will  be  prefer- 


APPENDIX  487 

able  to  buy  prepared  specimens.  These  may  be  obtained  at 
slight  expense  from  dealers  in  microscopic  supplies.  Such 
specimens,  as  well  as  sections  of  various  organs,  are  very  neces- 
sary, as  it  is  only  by  a  clear  comprehension  of  the  structure  of 
the  different  tissues  and  of  the  organs  which  they  compose  that 
the  student  can  understand  the  functions  of  the  various  parts. 


CHAPTER  XIII 

The  principal  chemical  changes  taking  place  during  digestion 
in  the  higher  animals  may  be  illustrated  with  very  simple  appa- 
ratus, and  at  the  cost  of  but  little  time.  It  is  not  necessary  that 
the  student  possess  any  knowledge  of  chemistry.  The  object 
of  digestion,  viz.,  the  changing  of  substances  which  are  in- 
capable of  absorption  into  substances  which  may  be  absorbed, 
can  be  made  plain  even  to  the  youngest  student.  The  chemi- 
cals needed  may  be  obtained  of  any  druggist. 

The"  following  experiments  deal  with  the  three  principal  di- 
gestive fluids,  viz.,  saliva,  gastric  juice,  and  pancreatic  juice ; 
and  with  the  main  kinds  of  foods,  i.e.,  starchy,  albuminous, 
and  fatty  substances. 

SALIVARY  DIGESTION 

(i)   The  microscopical  appearance  of  undigested  starch  and  its 
reaction  with  iodine 

Into  a  test  tube  about  one  fourth  full  of  water  put  a  pinch  of 
corn  starch  and  shake  the  tube.  Notice  that  the  starch  does 
not  dissolve.  Examine  a  drop  of  the  mixture  under  a  micro- 
scope and  note  the  starch  grains  floating  about  in  the  water. 
Add  a  drop  or  two  of  dilute  iodine  solution  to  the  mixture  in 
the  tube  and  note  that  it  turns  a  deep  blue.  Examine  a  drop 
of  this  mixture  under  the  microscope  and  note  that  each  starch 
grain  has  turned  blue. 

Prepare  another  test  tube  with  water  and  starch,  and  boil  the 
mixture  in  the  flame  of  an  alcohol  lamp  or  of  a  Bunsen  burner, 
keeping  the  tube  agitated  all  the  time  in  order  to  prevent  the 


488  APPENDIX 

starch  from  sticking  to  the  inside  of  the  tube.  Note  that  the 
starch  swells  up  and  forms  a  paste,  but  does  not  actually  dissolve. 
Cool  the  paste  by  holding  the  test  tube  in  cold  water.  When 
sufficiently  cool  add  a  drop  or  two  of  iodine  and  note  that  the 
starch  turns  blue.  This  change  of  color  serves  as  a  test  for 
starch  whether  uncooked  or  cooked.  Hence  we  see  that  undi- 
gested starch  is  in  the  form  of  granules  which  do  not  dissolve 
in  water,  but  which  turn  blue  when  treated  with  iodine. 

(2)    The  chemical  test  for  digested  starch,  i.e.,  grape  sugar 

Into  a  test  tube  about  one  fourth  full  of  water  put  a  pinch  of 
grape  sugar,  shake  the  tube,  and  npte  that  the  grape  sugar  dis- 
solves. Test  the  solution  with  iodine  and  note  that  the  blue 
color  does  not  appear. 

Prepare  another  solution  and  to  it  add  about  one  fifth  its 
volume  of  a  strong  solution  of  sodium  hydrate,  then  to  this 
mixture  add  a  drop  or  so  of  a  one-per-cent  solution  of  cupric 
sulphate.  Shake  the  tube  to  mix  the  contents  thoroughly. 
Note  the  light  blue  color.  Boil  the  contents  of  the  tube  and 
the  color  changes,  varying  from  lignt  yellow  to  orange  or  brick 
red.  Hence  it  is  seen  that  digested  starch  (grape  sugar)  dis- 
solves in  water,  does  not  turn  blue  with  iodine,  but  turns  yellow 
or  reddish  when  boiled  with  a  mixture  of  sodium  hydrate  and 
cupric  sulphate. 

(3)   The  digestion  of  starch  by  saliva 

Collect  about  a  third  of  a  test  tube  full  of  saliva,  the  flow  of 
which  may  be  promoted  by  chewing  a  piece  of  rubber  or  a 
button.  Dip  a  piece  of  red  litmus  paper  into  the  saliva  and 
note  that  the  paper  becomes  faintly  blue,  indicating  that  the 
saliva  is  slightly  alkaline  in  its  chemical  reaction.  In  another 
test  tube  make  a  mixture  of  about  equal  parts  of  saliva  and 
water,  and  to  this  add  a  few  drops  of  cool  starch  paste.  Hold 
the  tube  containing  this  mixture  in  the  hand  for  five,  or  ten 
minutes  in  order  to  keep  it  at  the  temperature  of  the  body. 
After  a  few  minutes  pour  a  portion  of  the  mixture  in  another 
tube  and  test  with  iodine,  which  will  probably  give  the  blue 


APPENDIX  489 

color  indicating  the  presence  of  starch.  Pour  a  second  portion 
into  another  tube,  add  sodium  hydrate  and  copper  sulphate, 
and  boil.  If  the  yellow  color  appears  it  indicates  that  some 
of  the  starch  has  already  been  digested  by  the  saliva,  i.e.,  has 
been  changed  to  grape  sugar,  which  remains  dissolved  in  the 
fluid  in  the  test  tube.  If  the  yellow  color  does  not  appear  on 
the  first  trial,  make  another  after  an  interval  of  a  few  minutes. 

(4)    To  show  that  digested  starch  is  capable  of  absorption, 
while  undigested  starch  is  not 

Prepare  two  dialyzers.  The  parchment,  or  parchment  paper, 
which  in  each  dialyzer  separates  the  contents  of  the  inner  from 
the  contents  of  the  outer  jar,  may  be  considered  to  represent 
roughly  the  membrane  lining  the  alimentary  canal,  through 
which  membrane  substances  are  absorbed  into  the  system. 
Into  the  inner  jar  of  one  dialyzer  put  a  solution  of  grape  sugar ; 
into  the  inner  jar  of  the  other  put  some  thin  starch  paste. 
After  an  hour  or  two  test  the  water  in  the  outer  jar  of  the  first 
dialyzer  for  the  presence  of  grape  sugar :  that  in  the  outer  jar 
of  the  other  dialyzer  for  starch.  It  will  be  found  that  grape 
sugar — i.e.,  digested  starch — dialyzes,  while  undigested  starch 
does  not.  In  other  words,  undigested  starch  cannot  be  ab- 
sorbed. The  experiment  may  be  varied  by  putting  both  grape 
sugar  and  starch  paste  into  the  same  dialyzer.  Or,  a  mixture 
of  starch  paste  and  saliva  may  be  put  into  the  one,  while  starch 
paste  alone  is  put  into  the  other  dialyzer. 

GASTRIC  DIGESTION 

(i)   Some  of  the  chemical  reactions  of  undigested  albuminous 
substances  (proteids) 

Into  a  bowl  or  beaker  break  the  white  of  an  egg,  cut  it  to 
pieces  with  a  pair  of  scissors,  add  fifteen  or  twenty  times  its 
bulk  of  water,  mix  thoroughly  by  stirring,  but  do  not  beat  it, 
then  strain  through  muslin  to  remove  the  fine  flakes  of  coagu- 
lated matter. 


490  APPENDIX 

(a)  Fill  a  test  tube  one  fourth  full  of  the  mixture  and  boil. 
The  albumen  coagulates. 

(<£)  Prepare  another  tube  and  add  a  few  drops  of  nitric  acid. 
The  albumen  coagulates.  Boil.  The  coagulated  mass  turns 
yellow.  Cool  the  tube  and  add  ammonia.  The  color  deepens 
to  orange. 

(V)  Prepare  another  tube  and  add  a  few  drops  of  Millon's 
reagent.  The  albumen  is  coagulated,  and,  on  boiling,  turns  red- 
dish. If  only  a  little  proteid  is  present  no  coagulation  will  occur, 
but  the  mixture  will  redden  when  boiled. 

(d)  Make  the  contents  of  another  tube  strongly  acid  with 
acetic  acid,  then  add  a  few  drops  of  potassium  ferrocyanide, 
and  a  white  precipitate  will  form. 

(2)  Some  of  the  chemical  reactions  of  digested  pro  teids 
{peptones} 

Make  a  peptone  solution  by  dissolving  some  of  Merck's  pep- 
tone in  water.  Repeat  the  experiments  given  for  proteids. 
Results  similar  to  those  in  (b}  and  (c)  will  be  obtained,  but 
the  peptone  does,  not  coagulate  on  boiling,  nor  does  it  give  the 
white  precipitate  with  acetic  acid  and  potassium  ferrocyanide. 

(3)    To  show  that  peptones  are  diffusible  through  membranes, 
while  proteids  are  not 

Prepare  the  two  dialyzers  as  for  the  experiments  with  starch 
and  grape  sugar.  Into  the  inner  jar  of  one  dialyzer  put  some 
of  the  white-of-egg  mixture,  and  into  the  other  some  peptone 
solution.  After  a  few  hours  test  the  water  in  the  outer  jar  of 
each  dialyzer.  It  will  be  found  that  the  peptone  passes  through 
the  membrane,  while  the  proteid  does  not. 

(4)    To  show  that  the  gastric  juice  digests  proteids,  i.e., 
changes  them  to  peptones 

Prepare  some  artificial  gastric  juice  as  follows  :  Make  some  .2 
per  cent  hydrochloric  acid  by  mixing  5.5  cubic  centimeters 
of  hydrochloric  acid  (sp.  gr.  1.16)  with  enough  distilled  water 


APPENDIX  491 

to  make  one  liter.  In  100  cc.  of  this  acidulated  water  put  100 
milligrammes  of  a  6000  pepsin,  or  150  mg.  of  a  4000,  or  300 
of  a  2000  pepsin.  Any  commercial  pepsin  maybe  used.  Pre- 
pare the  proteid  by  boiling  an  egg,  and  then  cutting  the  white 
into  small  cubes  or  shreds.  In  place  of  the  boiled  egg  some 
of  Merck's  prepared  fibrin  may  be  used. 

With  litmus  paper  test  the  reaction  of  the  artificial  gastric 
juice.  It  will  turn  blue  litmus  paper  red,  thus  showing  that  its 
reaction  is  acid. 

Fill  a  test  tube  about  one  fourth  full  of  the  artificial  gastric 
juice,  and  add  a  few  pieces  of  coagulated  white  of  egg  or  of 
fibrin ;  then  set  the  tube  in  a  warm  place,  as  in  a  water  bath 
regulated  to  about  37°  C.,  or  near  a  stove.  Examine  the  tube 
from  time  to  time.  The  cubes  of  egg  will  be  seen  to  be  disinte- 
grating and  dissolving. 

A  quantity  of  digested  white  of  egg  may  be  prepared  in  a 
cup  or  bowl  and  emptied  into  the  inner  jar  of  a  dialyzer.  After 
a  time  the  water  in  the  outer  jar  will  give  the  peptone  tests, 
showing  that  the  digested  albumen  is  diffusible. 

PANCREATIC  DIGESTION 

Procure  some  of  the  commercial  pancreatic  preparations  and 
make  an  artificial  pancreatic  juice  according  to  the  directions 
furnished  with  each  preparation.  Test  the  reaction  with  litmus 
paper.  It  will  be  found  to  be  alkaline.  Try  the  effect  of  the 
artificial  preparation  on  starchy  and  on  albuminous  substances 
in  the  manner  given  above  for  each.  The  pancreatic  juice  will 
be,  found  to  change  starch  to  grape  sugar  and  proteids  to  pep- 
tones. Try  its  effect  also  on  oil  by  adding  a  few  drops  of  olive 
oil  to  some  pancreatic  juice  in  a  test  tube.  At  first  the  oil  will 
float  on  the  surface  of  the  liquid.  Shake  the  tube  vigorously 
to  mix  the  two  substances.  The  oil  will  be  broken  up  into  fine 
droplets,  giving  the  contents  of  the  tube  a  milky  appearance. 
On  standing  for  a  time  it  will  be  seen  that  the  oil  does  not 
separate  from  the  digestive  juice  and  collect  at  the  surface  as 
it  would  if  shaken  up  with  water,  but  the  two  fluids  remain 
intimately  mixed,  forming  an  emulsion.  Under  a  microscope 


492  APPENDIX 

examine  a  drop  of  the  emulsion.  It  will  be  seen  to  consist  of 
innumerable  fine  drops  of  oil,  which  remain  separate  from  one 
another.  If  oil  be  shaken  up  with  saliva  or  with  artificial  gastric 
juice  no  emulsion  will  be  formed,  the  oil  soon  separating. 


CHAPTER  XV 

Directions  for  obtaining  and  studying  blood  corpuscles  are 
given  in  the  notes  on  Chapter  VIII.  Sufficient  blood  to  show 
the  phenomena  of  clotting  may  be  obtained  by  chloroforming 
a  rabbit  or  a  fowl,  cutting  one  of  the  veins  in  the  neck,  and 
catching  the  blood  in  small  tumblers  or  beakers. 


CHAPTER   XVI 

The  beat  of  the  heart  is  very  conveniently  studied  in  the 
frog.  Put  a  live  frog  into  a  glass  bowl  with  a  piece  of  cotton 
batting  or  of  cloth  saturated  with  chloroform,  and  cover  the 
bowl.  In  a  few  minutes  the  animal  will  have  become  motion- 
less and  insensible.  Remove  it  from  the  bowl ;  with  a  sharp 
knife  divide  the  skin  and  cartilage  at  the  base  of  the  skull,  thus 
making  an  opening  into  the  brain  cavity ;  into  the  latter  thrust 
a  wire,  and  by  twisting  it  about  destroy  the  brain.  The  frog 
will  probably  struggle,  but  its  motions  are  reflex,  and  it  has  no 
consciousness  of  pain.  The  heart  may  now  be  exposed  by 
making  an  incision  through  the  skin  and  muscles  of  the  upper 
part  of  the  abdomen  and  removing  the  cartilaginous  part  of  the 
breastbone.  The  heart  will  be  seen  beating  inside  the  pericar- 
dium. The  latter  may  be  removed  and  the  heart  freely  exposed. 
After  studying  the  movements  of  the  organ  it  may  be  removed 
from  the  body  by  cutting  the  blood  vessels  close  to  their  junc- 
tion with  the  heart,  and  placed  on  a  plate  of  glass  or  in  a  watch 
glass  containing  .75  per  cent  salt  solution.  Its  movements  will 
continue  a  long  time  after  its  removal  from  the  body.  The 
organ  may  afterward  be  opened  and  the  relation  of  its  ventricle, 
auricles,  and  the  connecting  veins  and  arteries  studied  (Fig.  273). 


APPENDIX  493 

• 

The  heart  of  the  pig,  sheep,  or  calf  may  be  used  to  show  the 
structure  of  the  mammalian  heart.  It  is  best  to  procure  at  the 
meat  shop  several  "plucks,"  i.e.,  heart,  lungs,  and  trachea  all 
attached  together.  Instructions  should  be  given  the  butcher 
that  the  parts  are  to  be  left  intact,  otherwise  they  will  be  found 
to  be  punctured  with  knife  cuts.  Dissect  out  the  blood  vessels 
for  some  little  distance  from  the  heart  in  order  to  get  their  re- 
lations. Open  some  of  the  hearts  lengthwise,  others  crosswise, 
to  show  the  internal  structure  (Fig.  271).  Pour  water  into  the 
cavities  to  show  the  action  of  the  valves.  The  flow  of  blood 
through  the  heart  may  be  illustrated  by  connecting  the  aorta 
with  the  venae  cavae  by  means  of  rubber  or  glass  tubing  to 
represent  the  systemic  circulation,  and  the  pulmonary  artery 
with  the  pulmonary  veins  to  represent  the  pulmonary  circula- 
tioivthen  rilling  the  heart  with  water  or  a  colored  fluid  and 
compressing  the  organ  with  the  hand  (Fig.  273). 

The  circulation  may  be  studied  in  the  web  of  the  frog's  hind 
foot.  Procure  a  thin  board  large  enough  to  lay  the  frog  upon ; 
in  one  end  make  a  hole  about  a  half-inch  in  diameter,  over 
which  the  web  may  be  stretched;  anaesthetize  the  frog  with 
ether  or  chloroform ;  as  soon  as  the  animal  becomes  insensible 
lay  it  on  the  board,  with  its  body  covered  with  a  moist  cloth ; 
over  the  larger  toes  of  the  foot  to  be  examined  slip  nooses  of 
thread,  and  fasten  these  in  slits  around  the  edge  of  the  board 
in  such  positions  as  to  spread  the  web  between  two  of  the  toes 
over  the  hole  in  the  board.  Put  a  drop  of  water  on  the  web,  lay 
on  the  cover  glass,  place  the  board  on  the  microscope,  and  ex- 
amine with  a  one  fifth  or  a  one  sixth  objective.  The  anaesthetic 
must  be  renewed  from  time  to  time,  otherwise  the  struggles  of 
the  animal  will  interfere  with  observation  (Fig.  263). 


CHAPTER   XVII 

The  gross  structure  of  the  frog's  lung  may  be  studied  in 
specimens  which  have  been  removed  from  the  body,  inflated 
with  air  blown  through  a  small  glass  tube  inserted  through  the 
glottis,  and  placed  in  alcohol  a  few  hours  to  harden.  When 


494  APPENDIX 

cut  open  the  lung  will  be  seen  to  be  a  hollow  sac  with  corru- 
gated walls  (Fig.  282). 

"  Plucks  "  obtained  from  a  butcher  will  illustrate  the  struc- 
ture of  the  mammalian  larynx,  trachea,  bronchial  tubes,  etc.  If 
fresh  and  not  punctured  with  the  knife  they  may  be  inflated. 
To  work  well  they  should  be  kept  moistened  (Fig.  283). 

The  presence  of  carbon  dioxide  in  the  air  exhaled  from  the 
lungs  may  be  shown  by  using  limewater  or  baryta  water,  with 
either  of  which  carbon  dioxide  forms  an  insoluble  precipitate, 
which  at  first  floats  as  a  delicate  white  film  on  the  surface  of 
the  liquid.  Pour  some  of  the  fluid  into  a  saucer  or  watch  glass, 
then  breathe  heavily  upon  it  a  few  times  through  the  mouth, 
and  the  film  will  be  formed. 


CHAPTER  XVIII 

The  structure  of  the  kidneys  is  well  illustrated  by  the  kidney 
of  the  sheep.  Several  of  these  should  be  procured  and  opened 
in  various  directions  to  show  the  structure  (Fig.  290). 


CHAPTER  XIX 

With  little  trouble  skeletons  of  frogs,  birds,  and  mammals 
with  bones  connected  by  flexible  attachments  may  be  prepared. 
Carefully  cut  away  all  of  the  muscles  and  other  soft  parts,  leav- 
ing only  the  ligaments  connecting  the  bones.  Then  place  the 
roughly  prepared  specimen  for  one  or  two  weeks  in  Wicker- 
sheimer's  fluid,  which  is  prepared  as  follows  :  In  three  liters  of 
boiling  water  dissolve  100  grams  of  alum,  60  grams  of  caustic 
potash,  25  grams  of  salt,  12  grams  of  saltpeter,  and  10  grams 
of  arsenic.  Cool  and  filter  the  liquid.  Then  to  each  liter  of 
the  fluid  add  400  cubic  .centimeters  of  glycerine  and  100  cubic 
centimeters  of  alcohol.  The  ligaments  of  skeletons  soaked  in 
this  fluid  will  remain  flexible  during  many  months  of  exposure 
to  the  air.  Should  the  ligaments  become  stiffened,  their  flexi- 
bility may  be  restored  by  a  few  hours'  immersion  in  the  fluid. 


APPENDIX  495 

CHAPTER   XX 

Muscle  fibers  for  microscopic  examination  may  be  obtained 
from  the  leg  of  a  frog,  or  even  from  the  body  of  a  recently 
killed  animal  at  the  meat  shop.  Lay  a  small  piece  of  muscle 
in  a  drop  of  .75  per  cent  salt  solution  on  a  glass  slide,  and 
with  a  pair  of  dissecting  needles  carefully  pick  the  muscle  to 
pieces.  Some  of  the  smallest  shreds,  upon  examination  with  a 
one-fourth  or  a  one-sixth  inch  objective,  will  be  seen  to  be 
single  or  grouped  muscle  fibers,  which  will  show  the  striations 
and  the  sarcolemma  (Fig.  208). 

CHAPTER  XXI 

Nerve  fibers  are  readily  obtained  from  the  sciatic  nerve  in 
the  frog.  This  nerve  may  be  found  by  removing  the  skin  from 
the  back  of  a  frog's  thigh  and  carefully  separating  the  under- 
lying muscles.  Among  them  will  be  seen  the  sciatic  nerve, 
covered  in  places  with  dark  gray  or  black  pigment  spots. 
Remove  a  quarter  to  a  half  inch  of  the  nerve,  being  careful  to 
stretch  it  as  little  as  possible ;  lay  it  on  the  glass  slide  in  a  few 
drops  of  .75  per  cent  salt  solution ;  cautiously  tear  it  to  pieces 
in  the  direction  of  its  length  with  dissecting  needles ;  then  put 
on  a  cover  glass  and  examine  under  a  high  power.  The  nerve 
will  be  found  to  consist  of  a  number  of  nerve  fibers,  some  of 
which  will  show  the  primitive  sheath  (neurilemma),  medullary 
sheath,  and  axis  cylinder  (Figs.  210,  211). 

The  relation  between  the  stimulation  of  a  nerve  and  the  con- 
traction of  the  muscle  to  which  the  nerve  runs  may  be  shown 
as  follows :  Expose  the  sciatic  nerve  as  directed  above ;  then 
with  the  quick  stroke  of  a  sharp  scalpel  sever  the  upper  end  of 
the  nerve  as  near  the  body  as  possible.  At  the  moment  of  do- 
ing this  the  muscles  of  the  leg  and  foot  will  probably  contract. 
Allow  the  nerve  to  rest  for  a  few  minutes ;  then  pinch  its  upper 
end  with  a  pair  of  forceps.  Again  the  muscles  will  contract. 
The  stimulation  may  be  repeated  at  intervals  if  the  nerve  be 
allowed  to  rest  for  a  few  minutes  between  successive  stimula- 


496  APPENDIX 

tions.  Try  also  the  effect  of  touching  the  nerve  with  a  hot 
wire  and  with  a  drop  of  dilute  acid  or  alkali.  During  experi- 
mentation the  nerve  preparation  must  be  kept  moistened  with 
the  salt  solution. 


CHAPTER   XXIII 

The  structure  of  the  egg  may  be  studied  in  the  starfish  or 
sea  urchin,  frog  or  fowl.  Starfish  eggs  preserved  in  various 
stages  of  segmentation  may  be  purchased  from  the  Department 
of  Laboratory  Supply  of  the  Marine  Biological  Laboratory  at 
Wood's  Hole,  Mass.  Frogs'  eggs  may  be  found  in  ponds  and 
ditches  in  early  spring.  If  transferred  to  the  laboratory  and 
kept  supplied  with  fresh  water  they  may  be  watched  through 
their  various  stages  of  segmentation  to  the  formation  of  the 
tadpole,  its  liberation  from  the  egg,  and  its  later  development. 
Compare  with  Fig.  370.  To  watch  the  development  of  a  chick, 
eggs  may  be  incubated  by  a  hen  or  in  an  artificial  incubator,  one 
egg  being  removed  each  day,  and  opened  by  breaking  away  a  cir- 
cular piece  of  the  shell  on  the  upper  side.  If  kept  submerged  in 
a  dish  of  .75  per  cent  salt  solution,  warmed  to  the  temperature 
of  the  body,  the  embryo  chick  may  be  kept  alive  for  several 
hours  to  show  the  beating  of  the  heart,  etc.  (Figs.  365,  366). 


INDEX 


In  the  Index  the  numbers  in  Roman  type  (289)  refer  to  pages  ;  those  in  bold-faced 
type  (254)  refer  to  cuts. 


ABOMASUS 291,  354 

Absorption '297,  256-258 

Acaleph 73,  374 

Acarida 123,  83 

Accipitres 166 

Acetabulum 356 

Acineta 12 

Acipenser 125 

Acorn  shell 102,  58 

Acrania 141,  144.  117 

Actinia 256,  25,  26,  236 

Actinophrys  sol 59 

Actinozoa ,  "...    .      lilt 

Adder .  134 

Adelochorda 140,  141 

Adipose  tissue     ......    236,  207 

Molis, 130 

^Epyornis    ....,"> 163 

Alaus .    .    ,     .     .    117 

Albatross 165 

Albumen     «• .    215 

Alcyonaria 75,  27,  34,  35 

Alimentary  canal     276,  414,  53,  164,  165 

Allantoidea 470 

Allantois 324,  414,  365-367 

Alligator 159,  138,  377 

Allolobophora 95 

Alternate  generation     .     .     .  33,  424,  374 

Amblypterus,  scale  of 119 

Ambulacra 92,  339 

Ameiurus 146,  126 

Ammonites 134 

Amnion 413,  414,  366 

Amoeba   .     .    256,  276,  335,  364,  366,  377,  1 
Amphibia    .     .    141,  151,  405,  407,  421,  426 

Amphiccelous 477 

Amphioxus       .     .     141,  144,  251,  301,  117 

Amphithoe  maculata 62 

Anallantoidea 470 

Analogy 429 

Anasa 112 

Anas  boschas 145 

Anatomy     . 12 

Anchylosed 353 

Animal  life  .  .     242 


Animalcule,  see  Protozoa. 

Annelides 95 

Annulata 50,  82,  95 

Anodonta 127,  275 

see  Clam. 

Anseres 166 

Ant 110,  119,  120,  398 

Ant-eater     .     .     .      179,  181,  290,  401,  168 

Antennae 387,  344 

Anthophora  retusa 22O 

Anthropopithecus  troglodytes 

189,  233,  317 

Anura 152 

Aorta 309,313 

Ape 193,  270,  290,  383 

Aphis 112 

Apis 119,  79,  24O 

Aplysia 130,  331 

Aptenodytes  pennantii      .....  142 

Apteryx 162 

Aquila  chrysaetos 148 

Arachnida 120,  261 

see  Centipede,  Scorpion,  Spider. 

Araneida 121 

see  Spider. 

Arbacia 92 

Arcella    . 56 

Archseopteryx 463 

Ardea 144 

Arenicola 95,  274 

Areolar  tissue 233,  2O7 

Argonauta 135,  109 

Argynnis 115 

Armadillo    .     .     .   181,  344,  401,  169,  298 

Artemia 101 

Artery 309,  265 

Arthropoda,     50,  97,  253,  274,  336,  346,  53 

digestive  process  of 295 

nervous  system  of 378 

number  of 433 

see  Crab,  Insecta,  Lobster,  Myri- 
apoda,  Spider. 

Arthrostraca 104 

Ascidian 142,  313,  114 

Aspidobranchia 129 


497 


498 


INDEX 


Aspidonectes 

Assimilation 

Astacus  fluviatilis     ...... 

Asterias 

Asteroidea 

see  Starfish. 

Astraea    . 76,  77, 

Astrophyton 

Atavism 

Ateles 

Atlas 

Attacus  pavonia-major 

Auger  shell 

Auk 

Aurelia 

Aves .m 

Avocet 

Axis . 

Axolotl  . 


.  157 
.  245 
.  54 

90 


3O,  32 

.  91 
.  428 
.  317 
.  354 
.  73 
.  98 
.  170 
.  73 
141, 159 
.  170 
.  354 
48 


BABIRUSA    v 

Baboon 

Badger 

Balaena 

Balaenoptera 

Balanoglossus  . 


232 

199 

190 

171,311 

228 
140,  141,  142,  113 


Balanus  .........       102,  58 

Bandicoot    ..........    180 

Barb   ............    346 

Barbule   ...........    346 

Barnacle      .....      258,  423,  428,  57 

see  Cirripedia. 
Barn  owl      ..........  155 

Basket  fish  ..........      91 

Bat      ...   192,  370,  401,  184,  185,  378 

Batrachian,  see  Anura,  Frog,  Toad. 

Bear    .     .    .....   190,  373,  401,  325 

Beaver    ....     191,  344,  397,  398,  182 

Bedbug  ...........    112 

Bee,     119,  250,  280,  328,  399,  406,  417,  278 
alimentary  canal  of  ......  24O 

head  of  ........  22O,  352 

instinct  of  .......      397,398 

muscle  in    .........    365 

respiration  in  ......      321,  327 

see  Hymenoptera,  Insecta. 
Beetle      .....  116,  117,  254,  369,  64 

alimentary  canal  of  ......  239 

eye  of  ...........  353 

see  Coleoptera,  Insecta. 
Belemnites  ..........    135 

Bellbird  ...........    400 

Beroe  ............      81 

Bibio  febrilis    .........  324 

Bile     ............    295 

Biology  ...........      11 

Bird,  159,  163,  250,  354,  356,  405,  406,  407, 

410,  422,  426,  427,  429 

alimentary  canal  of  ......    286 

auditory  passage  of  ......    391 


BIRD 

beak  of 344 

brain  of 380,  381,  384 

blood  of 303 

bones  of 346 

circulation  in  ....       314,  316,  273 

eggs  of 419 

eyes  of 395 

feathers  of 337 

flight  of 370 

instinct  of 397 

intelligence  of 398 

jaws  of 263 

locomotion  of 375 

lung  of 279 

muscle  in 364,  366 

parts  of 139 

pelvic  arch  of 356 

respiration  in 323-327 

scales  of 344 

skeleton  of 346 

temperature  of 327 

vocal  organs  of    ." 400 

wings  of 369 

Bittern 165 

Bivalves 256,  341,  296 

see  Clam,  Lamellibranch,  Oyster. 

Blackbird 176 

Blastoderm 417 

Blastula 410,  361  c 

Blood  .     .    301,  302,  306,  308,  309,  316,  318 

corpuscles 259 

of  fishes 147 

of  frog 26O-262 

Blubber 182 

Bluefish 12O 

Boa  constrictor    .     .     .   255,  275,  348,  235 

Bombus 119 

Bombyx .    116 

Bones      ....  234,  346,  356,  2O3,  204 

of  human  skull 2O5 

of  the  mammalian  skull   ....    351 

Bos  taurus 315 

Bot  fly 1 13,  256 

Brachiopoda     .....     82,  86,  43,  44 

Brachycephalic 470 

Brain 147,  380,  335-342,  348 

Brain  coral       31 

Branchipus 101 

Branta  canadensis    ........  146 

Brittle  star 91 

Bronchi 326 

Bubble  shell 180,  91 

Buccinum 129,58,88,227 

Budding 402 

Bufo 153 

see  Toad. 

Bugs 112 

see  Hemiptera. 
Bulimus 118,  93 


INDEX 


499 


Bulla  .... 

Bullfrog 

Bullhead 

Butterfly      .     . 
milkweed 
proboscis  of 
tortoise-shell 

Byssus 

CADDIS  FLY 
Cake  urchin 
Cambarus 
Camel 
Cameo  shell 


130,  91 
131 

146 

118,  250,  369,  396,  429 
368 


72 
..    126 

110 
92 

103 

186,291 
129 


Campanularian  hydroid    .....     2O 

Canaliculi    ..........    235 

Canines  .........    271,  232 

Canthocamptus    ........  373 

Capillaries  .....  298,  309,  263,  265 

Caprimulgus    .........  157 

Capybara     ........    191,  180 

Carapace      ........     157,  312 

Carcharias  vulgaris  .     .     .....  122 

Carchesium      .........      63 

Cardium  costatum    .....       127,  87 

Carinatae      ..........    163 

Carnivora    .....   188,290,419,325 

Carp   .     .    .     149,  268,  284,  328,  246,  299 
Carpocapsa  pomonella  ......     74 

Carpus    ...........    355 

Carrion  beetles    ......     .     .    117 

Cartilage,  hyaline     .......  202 

Cassis     .........       129,  97 

Cassowary  ..........    162 

Castor  canadensis     .......  182 

Casuarius    ..........    162 

Cat     .....       190,256,328,336,387 

brain  of  ........    380,  339 

eyes  of   ..........    395 

intestine  of     ........  256 

molars  of    .........    271 

Caterpillar,  113,  334,  365,  371,  392,  422,  429 
anatomy  of     ........  238 

head  of  ..........     75 

nervous  system  of  ......  333 

Catfish    ...........  126 

Cebus  hypoleucus    .......  187 

Cecidomyia      .........    113 

Cell     ....    228,  198,  199,  2O7,  357 

Cement  .......       235,  267,  206 

Centipede     106,  258,  322,  367,  371,  378,  8O 
Centrum     .........    349,  304 

Cephalizatlon  .........    437 

Cephalodiscus      ........    140 

Cephalopoda   .....       134,  405,  245 

see  Cuttlefish.  Squid. 
Cephalothorax     .......  99,  340 

Cerambycidse  .........    117 

Ceratodus    ..........    151 

Cere    ...  .166 


Cerebellum 380,  334-342 

Cerebrum 380,  314-342 

Cervus  elaphus 174 

Ceryle 158 

Cetacea 182 

see  Whale. 

Chyetopoda 95 

Chalaza 404,  358 

Chameleon       155,  263 

Cheiroptera 192 

Chelae loo 

Chelonia      .' 157,  136 

see  Turtle. 

Chelydra 158 

Chilopoda 105 

Chimpanzee     .     196,  189,  191,  233,  317 

Chitin 340 

Chiton 127,  1OO 

Chorion 414 

Choroid  membrane  .     ....      391,394 

Chordata .137 

Chrysalis 481,  74 

Chrysaora 374 

Chyle 295,  307 

Chyme 295,296 

Cicada 112,  399,  67 

Cicatricula '.     .    404,  358 

Cicindela 117 

Cidaris,  spines  of 293,  294 

Cilia 63,  281,  363 

Ciliata     .     .     .    , 68 

Cimex 112 

Circulation    ....     245,  308,  112,  263 

Cirratulus  grandis 51 

Cirripedia 101,  57 

see  Barnacle. 

Civet 190 

Clam,  125,  127,  251,  282,  312,  371,  389,  332 

see  Lamellibranch,  Mollusca,  Oyster. 

Clamatores 175 

Classification    .     .  47,  51,  55,  201,  457,  197 

Claws 344 

Click  beetles 117 

Clio,  mouth  of 257 

Clisiopampa 116 

Cloaca 288 

Clothes  moth 116 

Clypeaster 92 

Coccinella 117 

Coccus 112 

Cochineal 112 

Cochlea   .     .     .     .     ; 390 

Cockatoo     . 172 

Cockchafer,  heart  of 266 

Cockle 127,87 

Cockroach HO 

Cod 146,  149,  380,  406,  127 

Codosiga 60,  6 

Cceca 278,  279 

Coecum    .  ...  ...  249 


5oo 


INDEX 


Ccelenterata 50,  68,  483 

Coenosarc .      77 

Coleoptera 116 

see  Beetle. 

Colias 115 

Columbae 171,  153 

Columella 128 

Condyle 353 

Cone  shell 129,99 

Confervas 219 

Conjugation 407 

Connective  tissue     .     .     .    232,  200,  301 

Cony 185 

Copepoda 101 

Coracoid 355 

Coral  ...      70,  74,  220,  337,  366,  27-35 

Corallite 77,  992 

Corallium 75,  76,  34,  81 

Cormorant 165,  286,  143 

Cornea 391,  354 

Corpus  callosum 177 

Corpuscles,  blood 302,  259 

see  Blood. 

Correlation 432 

Corydalis     ...........     110 

Cotalpa  lanigera 76 

Cow,  skeleton  of 315 

Cowry 129,94 

Coxa 372 

Crab 254,  389,  399,  405 

circulation  in 311 

legs  of 371 

mouth  of 258 

respiration  in  . 320 

shells  of. 337 

swimming 60 

teeth  of 265 

see  Lobster. 

Crane 169 

Crane  fly 113 

Crangon       103 

Craniata 138,  141,  144 

Crayfish 54,  55 

Cricket 110,  281,  429,  65 

Crinoidea 93,  366,  45,  5O 

Crocodilia    .     .    158,159,269,344,372,406 

mouth  of 224 

respiration  of 326 

skeleton  of 31O 

stomach  of 284,  247 

see  Keptilia. 

Crop 280,  239-241,  248 

Crow 176,  400 

Crustacea 98 

absorbent  system  of 297 

eyes  of 392 

cuticle  of 336 

gullet  of 279 

nervous  system  of 378 

shell  of  .    .  .    340 


CRUSTACEA 

see  Crab,  Lobster. 

Ctenactis  echinata 

Ctenophora ( 

Cuckoo    

Culex 113, 

Cupidonia  cupido 

Curculionidae 

Cursores 

Cuticle 

Cuttlefish,  135,  252,  343,  346,  405, 

brain  of  ....    *i    ... 

circulation  in 

eyes  of 

gizzard  of 

ink  of 

mouth  of 

nervous  system  of  .... 

pancreas  of      .     .         ... 

respiration  in 

tentacles  of 

see  Cephalopoda,  Sepia,  Squid. 

Cyanea . 

Cyclas 

Cyclops 

Cyclostoruata 

Cyprsea 

Cypris 

Cytherea  chione 


9,  81,  36 
.  .  173 
78,  369 
.  .  149 

.     118 


.  .  231 
410,  107 
.  .  348 

.    312 


334 
256 
378 


.  214 

.  73 
.  347 
.  56 

141,  144 

129,  94 

.     56 

.  296 


DADDY  LONGLEGS 113 

Daphnia 101,  56 

Dasypus 169 

Dasyurus 181 

Decapoda 102 

Decussate 395 

Deer 186,  174 

Deglutition .274 

Demodex  folliculorum 83 

Dendronotus  arborescens      .     .     .     .     9O 

Dental  formulas 185,  186,  271 

Dental  tissue 235 

Dentine 186,  235,  267,  206 

Development 409,  411,  416 

Devil's  darning  needle 110 

Diapheromera 110 

Diaphragm 289,  326,  285 

Diapophyses 352 

Dibranchs 134 

Didelphys  virginiana 167 

Didus 172 

Differentiation 227 

Difflugia 56 

Digestion     ....      245,  294,  248,  249 

Digitigrade 190,  373,  325 

Dinobryon 60,  5 

Dinornis 163 

Dinotherium 186 

Diplopoda 106 

Diploria  cerebriformis 31 


INDEX 


501 


Dipnoi 150 

Diptera 112 

see  Fly,  Mosquito. 

Discopora  skenei 43 

Distoma S3 

Distribution  of  animals     .     .     .    440  379 

Divers .         164 

Diving  beetles 117 

Dodo 172 

Dog 190,  250,  374,  387,  398 

artery  of 365 

brain  of 380,  381 

liver  of 389 

molars  of 271 

follicles  from  stomach  of  ....  387 
skull  of  ......     853,305-307 

Dogfish   . 148 

Dog  whelk 106 

Dolicocephalic 470 

Dolphin 183,  270,  173 

Donkey 381,  401 

Doris 130 

Dove 171,  250,  153 

Dragon  fly  .     . 369,  68 

Dromseus 162 

Duck,  wild 145 

Duck  mole 179,  267 

see  Ornithorhynchus. 

Dugong 185,  370 

Duodenum 293 

Dytiscus 117,  318,  334 

EAGLE 168,  148 

Ear 390,  347,  349 

Ear  shell 129,  1O6 

Earthworm,  95,  253,  279,  319,  328,  371,  378 

Echidna 179 

Echinarachnius 92 

Echinodermata     ....    50,  87,  433,  45 

Echinoidea 91 

Echinus 294,  45,  48,  337 

see  Sea  urchin. 

Ectoderm 66,  68,  277,  363 

Ectosarc 276 

Edentata 181 

Eel 149 

Egg 148,  403,  406,  416 

embryo  in 365 

fertilization  of 408,  361 

of  hen 358 

of  shark 360 

of  sponge 405,  359 

section  of 363,  366 

segmentation  of 361 

Elasmobranchii 148 

see  Bay,  Shark. 
Elephant,  185, 250, 264, 272, 273, 388, 398, 401 

brain  of 380 

feet  of 336 

molars  of    .    .         271 


ELEPHANT 

sinuses  of 

skelefon  of 
Elk-horn  coral 
Elytra 
Embryo 
Embryology 
Emu 
Enamel 
Encephalon 
Endoderm 
Endopodite 
Endosarc 
Endoskeleton 
Entomostracan 
Ephemera 
Epiblast 
Epidermis 
Epiglottis     .. 
Epistylis 
Epithelium 
Equus  caballus 
Ermine  weasel 
Esophagus  .     . 
Eucope 
Euglena 
Euplectella 
Eulamellibranchia 
Eustachian  tube 
Eutheria 
Evolution 
Excretion 
Exopodite 
Exoskeleton 
Eye     ....      391 


288 


347 
316 

..'....      70 
116,369 
412,  365,  367 
12,  14,  409 
162 

235,  267,  339 
385 

68,  277,  335,  363 
....      99 
276 

335,  337,  346 
423,  373 
110 

410,  411,  365 
231 

326,  335,  349,  356 
63 

230,  199 
314 
177 

,  347-350,  353,  356 
69 

60,  391,  4 
66 
127 
39O 
181 
450 

245,329 
99 

335,  337 
,  416,  35O,  353,  354 


FACIAL  ANGLE    ........    140 

Falcon     ...........    168 

Family    ...........      51 

Fasciculi      ..........    236 

Fat     ..........    236,  3O7 

Feather   ....     337,  345,  369,  415,  3O3 

Feather  star     .........      93 

Feet    ........  334,  335,  336 

Felis  domestica    ........  339 

Felisleo  ...........  303 

Femur    .........      356,  372 

Fertilization     .......      408,409 

Fiber  ........     ....     233 

muscular    .........  309 

nerve      „    .......    238,  31O 

Fibrin     .......  ......    302 

Filibranchia     .........    127 

Finch      ...........    176 

Fish   ............    145 

alimentary  canal  of      .....    284 

blood  of  .........  147,303 

circulation  in  ....    313,  368,  373 

eggs  of  ..........    148 

epidermis  of    ........    33T 


502 


INDEX 


FISH 

eyes  of 
fins  of 


gills  of 

heart  of  

locomotion  of 

mouth  of 

muscles  of 

pancreas  of 

respiration  in  .     .     .      320, 

scales  of 

skeleton  of 

skin  of   

teeth  of  

vertebral  column  of      .     . 

Fishhawk 

Fission 

Fissurella  lister! 

Flagella 

Flamingo     ....... 

Flatfish,  eyes  of 

Flea,  sand 

water 

Flight 

Flounder     

Fluke      

Fly 112,  324, 

metamorphosis  of   ... 

mouth  of 

see  Diptera,  Mosquito. 

Flycatcher  

Flying  fox 

Follicles 

Food .     .     . 

Foramen 

Foramina 

Foraminifera 

Formica 

Formicarium 

Forms  of  animals     .     .     .     . 

Fowl 

Fox 

Frigate  bird 

Frog, 

151,  323,  326,  337,  347,  348 

blood  of 

brain  of 

circulation  in 

corpuscle  of 

lungs  of 

metamorphosis  of    .     .     . 

skeleton  of 

teeth  of  

Fruit  moth 

Function 

Fungia 

Fusus -.     , 


...  395 
146,  368,  330 
...  370 
...  320 
...  313 
.  368,  331 
...  262 
.  364,  366 
...  381 
,  323,  324,  327 
.  343,  119 
.  346,  347 
...  399 
147,  268,  273 
...  352 
...  147 
...  402 
.  .  .  1O5 


165,  333 
.  .  395 
.  .  63 
.  .  56 

.     .    369 
.     149 


),  397,  399 
.  .  69 
.  .  333 


.  176,  159 
...  192 
.  329,  387 
...  245 
352,  353,  382 
...  58 
.  338,  313 
.  .  .  119 
...  120 
...  434 
.  364,  348 
.  190,178 
.  165 


374,  377,  419 
.  302,  36O 
.  384,  337 
.  313,  363 
.  .  .  361 
.  .  .383 
...  421 
...  384 
.  152,  269 
...  74 
...  240 
...  78 
.  129,  96 


GADUS  CALLARIAS 


137 


Gall  bladder 332,  389 

Gallfly 119 

Gallinae 168 

Ganglion 876,  313,  343 

Gannet 165 

Ganoid 149 

Garpike 149,  134 

Gastric,  follicles 330,  387 

juice 287 

teeth 265 

Gastropoda 127,  320 

anatomy  of 243 

teeth  of .337 

see  Snail. 

Gastrula 410,  363 

Gavise 170 

Gavial 159 

Gecko 154 

Gelatin 233 

Gemmules 68 

Genus 51 

Geometrids 116 

Germinal  vesicle 403 

Gills 147,  319,  346 

Giraffe 401 

Gizzard 280 

Gland 330,  386 

Glenoid  cavity 354 

Globigerina  ooze 59 

Glottis 326 

Glyptodon 181 

Gnawers 190 

Goatsucker 173,  157 

Goldsmith  beetle 76 

Goliath  beetle 117 

Goniaster 90, 46 

Goose 287,  146 

Gordius 84 

Gorgonia 75,  35 

Gorilla 196,  193,  193 

Grallse 169 

Granddaddy  longlegs 121 

Grantia 65,  66,  68,  14 

Grasshopper 110,  281,  389 

Gregarina 7,  8 

Ground  beetles 117 

Grouse 169 

Growth 425 

Gryllus HO,  65 

Guinea-pig 191 

Guii no 

Gymnophiona 153 

HADDOCK .1*9 

Haemocyanin 306 

Hagflsh 144,268 

Hair 337,  345,  3O1 

Hairworm 84 

Haliotis 129,  95,  106 

Hallux 161 


INDEX 


503 


Hare 191,  181 

Harpalus      . 107,64 

Harp  shell 129 

Harvest  fly .     .    399 

Harvest  man 121 

Haversian  canals 284,  304 

Hawk 168 

Hawk  moths 114 

Hawkbill  turtle 158,  136 

Hearing 889 

Heart 151,809,271 

of  cockchafer 266 

ofdugong 27O 

rudimentary 364 

single 369 

Hedgehog 192,345 

Heliozoa 59 

Helix 131,  92 

Helix  albolabris 318 

Helmet  shell 97 

Hemal  spine 352 

Hematocrya 470 

Hematotherma 470 

Hemiptera 112,  66 

Hemoglobin 306 

Hen's  egg 358 

Heredity 427,  455 

Hermit  crab 108 

Herodiones 165 

Heron 165,  144 

Herring 146,  149 

Hessian  fly 113 

Heterocercal 368 

Hippopotamus     ....      186,  373,  326 

Hirudinea 95 

Hirundo 163 

Histology 12,  14 

Hog 186,  264,  271,  333 

Holothuria 45,  49 

Holothuroidea 92 

Homarus  vulgaris 59 

Homocercal 368 

Homology 429,  375-378 

Homomorphic 428 

Honey-bag 280 

Honey  bee 119,  79 

Hoofed  mammals,  foot  of      ....  326 

Hoopoe 173 

Hornbill 173 

Horned  pout 126 

Hornera  lichenoides 42 

Horns 344 

Horny  sponge,  skeleton  of    ....     16 

Horse 185,  419,  427 

brain  of 380,381,335 

circulation  in 316 

foot  of 326 

forefoot  of 3OO 

intelligence>f 398 

locomotion  of  .  ,    373 


HORSE 

*    molars  of 271 

nostril,  skin  of 291 

skeleton  of 314 

skin  muscles  of 365 

skull  of 3O8 

speed  of 374 

stomach  of 290,  251 

teeth  of 185 

toes  of    .     .     . 356 

Horsefly,  mouth  of 222 

Horse-hair  snake 84 

Horseshoe  crab 123,  258,  336 

Housefly     ......'..      113,824 

Humble  bees 119 

Humming  bird 173 

Hyalaea  tridentata 89 

Hyaline  cartilage 2O2 

Hydatina 4O 

Hydra,  277,  335,  377,  410,  425,  17,  18,  374 
Hydractinia     .........      70 

Hydroid , .    416,  19,  20 

Hydrozoa 69 

Hyena 190 

Hylodes 460 

Hymenoptera 118 


.......    354 

.    .     .      410,411,365 
.     185 


Hyoid  bone 
Hypoblast  . 
Hyrax  .  . 


IBIS 165 

Ichneumon  fly 119 

Ichthyopsida 141 

Ichthyosaurus 48,  159 

Idotea  robusta     .    .    .  .-.     .     .       104,  61 

Iguana 155 

Iguanodon 159 

Ilium 855 

Imago      ....     69,  74,  76,  368,  369 

Impennes    .     .     , 165 

Incisors 271 

Incus 890 

Individuality 432 

Infusoria 63.251,276,377 

absorbent  system  of 297 

cuticle  of 335 

digestive  process  of 295 

locomotion  of 367 

mouth  of 256,  277 

Ingestion 245 

Insecta   .    106,  405,  407,  416,  426,  429,  295 

absorbent  system  of 297 

antennae  of 344 

biting 254 

circulation  in 816 

eyes  of 392 

feet  of •.     .    .     .  324 

flight  of  . 370 

horny  crust  of 840 


504 


INDEX 


INSECTA 

instinct  of 

legs  of 372,328 

liver  of 331 

locomotion  of 367 

metamorphosis  of 419 

mouth  of 258 

muscle  of 365 

poison  of 334 

skeletons  of    ........    341 

spiracle  of 276 

touch  of .'  .     .     .     .     .     387 

tracheal  tube  of 277 

wings  of 369 

Insectivora 192 

Instinct  . 395,  396 

Integument     ...  * 335 

Intelligence 395,398 

Inter-ambulacra 339 

Intervertebral  foramen 852 

Intestine,  see  Alimentary  canal. 

Iris 394,  354 

Ischium 355 

Ivory 186,  235,  267 

Ixodes 123 

JACANA 171 

Jackal 190 

Jay 176 

Jellyfish 252,  424,  22 

blood  of. 301 

circulation  in 310 

digestive  sac  of 278 

locomotion  of 367 

mouth  of 256 

nervous  system  of 377 

poison  of 334 

see  Acaleph. 

Julus 106 

June  bug 76 

KANGAROO  .     .   180,  290,  292,  374,  401,  407 

Key -hole  limpet 1O5 

Kidney    .     .    .   333,  112,  244,  25O,  290 

King  crab 123,  258,  836 

Kingfisher 173,  158 

Kite 168 

Kiwikiwi 162 

Klossia    .  .     .       7 


LABIUM 

Labrum 

Labyrinth 

Lacerta 

Lacertilia 

Lachnosterna  fusca 


254,  259,  319 

259,  319,  22O 

390 

133 

154 

76 


Lacteal    ....    .....    298,267 

Lacuna?  .......      235,  267,  2O5 

Lady-bird    ..........    117 

Lagena  striata      ........       2 


Lamellibranch      .    125,  126,  86,  244,  275 
see  Clam. 

Laminae       235 

Lamprey 144,  428,  118 

Lamp  shell 86 

Lancelet 141,  144,  117 

Land  snail 93 

Lark 176 

Larva 76,  77,  368,  369 

Larynx 356 

Lasso  cells 252 

Leech      .     .    95,  97,  250,  258,  279,  319,  371 

Lemur 193,  186 

Leopard 190 

Lepas  anatifera 57 

Lepidoptera     ........  113,  7O 

see  Butterfly. 

Lepidosiren 151 

Lepidosteus  osseus 134 

Libellula 110,  68 

Life 225,  242 

duration  of 437 

origin  of 219 

struggle  for     ........    438 

Lightning  bug 117 

Ligula 107,  259,  64,  33O 

Likeness 427 

Limax 131,  93 

Limbs 355,  375-378 

Limicolae 170 

Limnaea 134,  93 

Limpet 105,  337 

Limulus 123,124,258,336 

Lion 190,  256,  400,  417 

brain  of 384 

feet  of 335 

intestines  of 292 

skeleton  of 347,  303 

stomach  of 353 

Liver  ...     .  280,  330,  331,  113,  117,  389 
Lizard,  154, 155,  313,  326, 337, 373, 133,  338 

see  Lacertilia. 
Lobster, 

103,  254,  340,  405,  407,  422,  423,  59 

auditory  sacs  of 389 

circulation  in 311,  367 

eyes  of 392 

gullet  of 279 

locomotion  of 367 

mouth  of 258 

muscle  of 365 

respiration  in 320 

shell  of 339 

teeth  of 265 

Lobworm 95,  374 

Locomotion 366,  331 

Locust 110,  254,  281,  319 

Loligo 135,  1O8 

see  Squid. 
Long-horned  beetle 117 


INDEX 


505 


Loon 141 

Lophophore     ........    86,  41 

Louse 104,  112,  250,  420 

Lucernaria 73,  24 

Lumbricus 95 

see  Earthworm. 

Lungfish 151 

Lungs 321, 333 

Lupus  occidentalis 176 

Lymph 299 

Lymphatics 298,  258 

MACAW 172 

Mackerel 149 

Mactra 86,  244 

Madrepora 76,  78,  28,  33 

Magpie 400 

Malars 353 

Malleus 390 

Mammalia 177 

brain  of 380,  882 

circulation  in 314,  873 

dentition  of 271 

ear  of 391 

eggs  of 419 

hair  of 337,  345 

lacteal  system  of* 257 

locomotion  of .    •     .     .     .     .     .     .373 

lungs  of 283 

milk  of 334 

mouth  of 263 

muscle  of .      . 364 

pelvic  arch  of 356 

respiration  in 324,  326 

skin  of 336 

stomach  of 289 

temperature  of 327 

teeth  of 270 

touch  of 387 

Mammalian  vertebrate,  section  of  .     .  25O 

Man 193,256 

arm  and  leg  of 375 

brain  of  . 382,  383,  385 

digestive  apparatus  of .....  249 

intelligence  of 399 

intestines  of 292 

locomotion  of 367,  375 

molars  of 271 

metamorphosis  of 419 

nervDus  system  of 377 

organ  of  hearing  of 390 

pancreas  of 288 

skeleton  of.    ." 191 

teeth  of 199,270 

temperature  of 328 

toes  of 356 

tongue  of 225 

touch  of 387 

voice  of 401 

Manatee  .  .172 


Mandible,  254, 259, 354,  216, 22O,  222, 223 

Mantis 254 

Mantle 336,  244 

Manyplies 291,  254 

Marine  worm 51 

Marsh  hen 150 

Marsupialia '     180,  407 

Mason  spiders 123 

Mastigophora 60 

Mastodon 186 

Matrix 230 

Maxillae 254,259,819 

May  fly 110,  256 

Meandrina 77 

Medulla  oblongata,  380,  382,  384,  334-341 

Medullary,  furrows 411 

sheath 239,  21O,  211 

Medusa 70,  278,  424,  21 

see  Jellyfish. 

Megalosaurus 159 

Megatherium 181 

Melanoplus 110 

Meleagrina 127,  84 

Melolontha 328,  353 

Membrana  putaminis 403 

Mentum 64 

Mesentery  , 74, 286,  25 

Mesoblast 410,  411,  365 

Mesoderm   .     .    . 66,  68 

Mesothorax 341 

Metacarpus 855 

Metamorphosis     .     .      419,  421,  369,  37O 

Metatarsus 356 

Metatheria 180 

Metathorax 341 

Metazoa 51,  65 

Metridium 75 

see  Sea  anemone. 

Milkweed  butterfly 368 

Millepora 70 

Millipede 258,322 

see  Myriapoda. 

Mimicry 428 

Minerals  and  organized  bodies  .    .     .    215 

Mite 123,  83 

Moa 163 

Mockingbird 400 

Molar       .     .     .    271,  206,  229,  233,  234 

Mole 192 

Mollusca, 

50,  124,  251,  303,  391,  405,  428,  296 

absorbent  system  of 297 

circulation  in 311 

deglutition  in 274 

digestive  process  of 295 

ear  of 347 

epidermis  of 337 

liver  of 831 

mantle  of 336 

mouth  of 257 


5o6 


INDEX 


MOLLCSCA 

nervous  system  of 331 

number  of 433 

respiration  in 820,  327 

shells  of 341,342, 

stomach  of 282 

teeth  of 346 

see  Clam,  Cuttlefish,  Snail,  Squid. 

Molluscoida 82 

Molting 337,422 

Monkey  .    .     .    193,  256,  374,  381,  387,  398 

thumbless 217 

Monotremata 179,  180 

Morphology 12,  14 

Mosquito 113,  250,  78,  369 

Moth  ....  113,  407,  429,  71,  74,  241 
see  Lepidoptera.    * 

Mother  Carey's  chicken 165 

Mother-of-pearl 341 

Motion 242,363 

Mouse 191 

Mouths  of  animals 256 

Mucous  membrane 255 

Mulberry  mass 410 

Mullet 149 

Murex 129 

Musca 113,  324 

see  Diptera,  Fly,  House  fly. 

Muscle 364,  208,  327,  328 

Muscular  fiber      ....  2O9,  318,  319 

Muscular  tissue 236 

Mushroom  coral 78,  29 

Mussel 125,  126,  127,  341,  85 

Mya    .     .     .     . 127 

Mycetozoa .      59,  3 

Myenia 66 

Myriapoda 106,392 

see  Centipede. 

Myrmecophaga  jubata 168 

Mytilus  pellucidus 85 

Myxine 144,  268 

NAILS 344 

Narwhal 270 

Nassa  reticulata 106 

Natural  selection 439, 455 

Nauplius  of  Entomostracan  ....  373 
Nautilus  .     .     134-137,  282,  341,  109,  HO 

Necrophorus  vespillo 77 

Necturus 152,  129 

Nemathelminthes 82,  84 

Nematocyst 68 

Nereis 95,97,253,279,215 

Nerve,      238,  239,  376,  888,  210,  329,  346 
Nervous  system  .   139,  376,  386,  33O-333 

Nervous  tissue 237 

Neurapophyses 349 

Neurilemma 239,  210,  211 

Neurology 14 

Neuron    .  .  211 


Neuroptera 110 

see  Dragon  fly. 

Neuroskeleton 352 

Newt,  metamorphosis  of 370 

Noctiluca 60 

Notochord 411,  117,  363 

Notonecta 112,  66 

Nucleolus 228,  198 

Nucleus 228,  198 

Nudibranch 130 

Nutrition 242,  244 

Nymph 77 

OCEANITE8 165 

Ocelli       106,  891,  352 

Octopus 135 

Odontoid  process 354 

Odontophore 129 

(Estrus 113 

Olfactory  lobes     ....     382,  336-338 

Olfactory  nerves 388,  346 

Oligochaeta  . 97 

Olive  shell 129 

Oniscus 104 

Onycophora 104 

Operculum,  128, 147,  169,  321,  342,  88,  3O9 
Ophidia 155 

see  Snake. 

Ophiocoma  russei 47 

Ophiura  . 91,  45,  47 

Ophiuroidea 90 

Opisthobranchia 129 

Opisthocselous 477 

Opossum 180,  167 

Optic  lobes 382,  394,  336-338 

Orang-outang,  196,  340,    188,   190,   191 

Orchelimum 110 

Order 51 

Organ 240 

Organization 227 

Organ-pipe  coral 27 

Origin,  of  life 219 

of  species 450 

Oriole 176,429 

Ornithorhynchus      ...       179,  267,  166 

Orthoceras 134 

Orthognathous 470 

Orthoptera 109 

see  Grasshopper. 

Orycteropus 181 

Oscines  .    .- 176 

Osculum 65,  68, 13,  15 

Os  innominatum 355 

Osseous  tissue 234 

see  Bone. 

Ossification 415 

Ostracoda 101 

Ostrea 127 

see  Oyster. 
Ostrich 162,  287,  375,  14O 


INDEX 


SO/ 


Otariajubata 179 

Otoliths  .......    389,  347,  348 

Otter 190 

Oviparous 140 

Ovipositor 108,  295 

Owl 173,  395,  155 

Ox 185,  250,  256 

brain  of 384 

foot  of 326 

horns  of 344 

intestines  of 292 

locomotion  of 373 

papilla?  of 336 

teeth  of 186 

see  Ungulata. 

Oyster,  125,  126,  12T,  251,  406,  416,  422,  426 
alimentary  canal  of  ......  242 

circulation  in 311 

locomotion  of 366 

mouth  of 25T 

muscle  in 365 

pearl 84 

respiration  in 320 

see  Clam,  Lamellibranch. 


PAGTTBUS     . 
Palaemon 
Palatines      . 
Pallial  sinus 
Palpiform  organs 


.     .     103 

.     .    354 

126,  296 

.     82 


Palpus     ....    259,  46,  75,  230,  223 

Paludina 92,  1O4 

Pancreas      ......      330, 331,  288 

Pandion  haliaetus 147 

Pangenesis 455 

Pangolin 181 

Panther 190 

Paper  nautilus 135,  109 

Papilio 115 

Papillae    ....   264,  336,  38T,  225,  345 

Paraglossse 64,  220 

Paramecium    .     63,  64,  65,  229,  407,  9,  1O 
see  Infusoria. 

Paramylum  bodies 4 

Paroquet 172 

Parrot     ....     164,  172,  388,  400,  154 

Parrot  fish       268 

Partridge 169 

Passenger  pigeon      . 171 

Passeres 175 

Patella    ...      129,  356,  105,  227,  303 

Pavament  teeth 230 

Pearl  oyster 127,  84 

Pearly  ear  shell 95 

Pearly  nautilus 11O 

Pecten 127,  391,  35O 

Pectinibranchia 129 

Pedicellari* 279,  294 

Pediculus 112 

Pelagia  noctiluca      . 22 


Pelecypoda 125 

Pelican 165,  286 

Pelomyxa 57 

Pelvic  arch       355 

Pelvis 856 

Penguin       163,  165,  142 

Pennatula 75,  35 

Pentacrinus 93,  50 

Pepsin 295 

Peptone       295 

Perca  fluviatilis 3O9 

Perch  .     .     .  146,  149,  343,  119,  3O9,  336 

Periostium       346,355,366 

Periotic  bone 358 

Peripatus 104,  63 

Periplaneta 110 

Peristaltic  motion 292 

Peritoneum 292 

Petrel 165 

Petrified  tissue 234 

Petromyzon  marinus 118 

Phalacrocorax 143 

Phalanges 355,356 

Phalangida 121 

Phalangium 121 

Phalarope 170 

Pharyngobranchii 144 

Pharynx 274,  288,  52,  239 

Pheasant 169 

Phryganea  ..........    110 

Phyllopoda 101 

Physalia       23 

Physeter 17O 

see  Whale. 

Physiology 12,220 

Picarise 173 

Pieris 115 

Pig,  bronchial  twig  of 28O 

Pigeon 164,  171,  287 

Pike 147,149,268 

Pike  perch 320 

Pinnigrade       189, 373,  325 

Pisces 141,  145 

see  Fish. 

Placenta 407,  414 

Planaria 82,  38 

Planorbis 128,  134,  462,  92 

Plantigrade 189,  373,  325 

Plant  louse 112 

Plants  and  animals 217 

Plasma ' 302 

Plastron 157 

Platyhelminthes 82,  250,  37 

Platyonychus 6O 

Pleisiosaurus 159 

Pleurapophyses 352 

Pleurobrachia  pileus 36 

Plover 170 

Podocyrtis  schomburgkii       .     .    -.     .       2 
Poison  apparatus 231 


5o8 


INDEX 


Polyp 262,  335,  428 

absorbent  system  of 297 

blood  of .     .    301 

coral  of 338 

liver  of 331 

mouth  of 256 

respiration  in 319 

Polystomella  crispa 3 

Polyzoa 82,  220,  428,  41,  43 

Pomatomus  saltatrix 13O 

Pond  snails 92 

Pons  varolii 177 

Porcupine 191,345,401 

Porifera .49,  65 

see  Sponge. 

Porites 78 

Porpoise      .    .  183,  2W,  290,  388,  401,  353 

Portal  circulation 476 

Portuguese  man-of-war     .     .     .     .    70,  33 

Potato  worm 116 

Poulpe 135 

Poultry 169 

Prairie  chicken 149 

Prawn 103,423 

Prehension  of  food 250 

Premaxillae 354 

Primates 193 

Proboscis,  of  butterfly 331 

of  elephant 260,264 

Procaelous 477 

Procyon  lotor 175 

Prognathous 470 

Protamceba 56 

Protective  resemblance 429 

Proteus 152,  303 

Proteus  anguinus 133 

Prothorax 341 

Protista  .    .     .    . 218 

Protophyta 56 

Protoplasm      ....       23,  215,  219,  226 

Protopodite 99 

Protopterus 151,  138 

Prototheria 179 

Protozoa, 

49,  51,  54,  276,  301,  391,  407,  416,  423 

number  of 433 

respiration  in 319 

shells  of 338 

see  Amoeba,  Infusoria. 

Proventriculus 287 

Psalterium 291,  354 

Pseudo-lamellibranchia 127 

Pseudopodia    ....    56,  251,  363,  313 

Psittaci 172 

see  Parrot. 

Pterodactylus 159 

Pteropod 89 

Pubis  . 355 

Pulex 113 

Pulmonary  circulation 309 


Pulmonata 

Pulse 

Pupa 

Pupil 

Putorius  noveboracensis 


476 
76 

394 

177 


Pygopodes 164 

Pyloric  opening 284,  290 

Pyrophorus 117 

QUADRUMANA 471 

see  Monkey. 

Quahog 127 

Quill 346 

EACCOON 190,  175 

Eadiates 808 

Eadiolaria 59,  338,  3 

Eadius 355 

Eaia  clavata 133 

Eail      ....  * 169,  15O 

Eallus  elegans 15O 

Eana 131 

see  Frog. 

Eank  of  animals 435 

Eat      .     .  ' 191,  344 

Eatitae 162 

Eattlesnake 155, 844,  331 

Eay     .    .     .     147,  148,  268,  866,  119,  33O 

Eedstart '.    .     .  161 

Eepair 425 

Eeproduction 242,  402,  438 

Eeptilia 141,  154,  269,  405,  426 

blood  of 303 

brain  of 380,  382,  384 

circulation  in 313,  373 

gullet  of 284 

mouth  of 262 

muscle  in 364 

pelvic  arch  of 356 

respiration  in 323,  324 

scales  of 343 

teeth  of .     273 

see  Crocodile,  Lizard,  Snake,  Turtle. 

Eespiration 245,  246,  318 

Eete  mucosum 336 

Eeticulum 291 

Eetina .     .    391,  35O,  355 

Ehabdopleura 140 

Ehea 162 

Ehinoceros       .     .  185,  344,  373,  173,  336 

Ehizopoda 56,  3 

Eingdove 153 

Eock  shell 129 

Eodentia      .     .     .     190,  271,  290,  419,  180 

Eotifers 82,  85,  26G,  4O 

Eudimentary  organs 418 

Eumen 291,  354 

Euminants 186,  255 

feet  of 186 

molars  of 271 


INDEX 


509 


RUMINANTS 

stomach  of ... 
teeth  of  .... 
see  Ox,  Ungulata. 


291,  254 
.    270 


SACRUM ...    855 

Salamander 429,  130 

Saliva 295 

Salivary  glands 330 

Salmon    .     .     .  149,  269,  364,  368,  397,  121 

scale  of 343,  119 

Salmo  salar 121 

Salpa 143 

Samia,  wing  of 71 

Sand  dollar 92 

Sand  flea .62 

Sandpiper 152 

Sandworm        95 

Sapajou,  white-throated 187 

Sarcolemma 237, 415 

Sarcophaga  earn  aria 69 

Sarcoptes 123 

Saurian,  teeth  of 269 

Sauropsida 141 

Scales      .     .     .  146,  337,  343,  7O,  71,  119 

Scallop 127,  391 

Scapula 355 

Sclerobase 76,  81,338 

Scleroderm 76,  77,  338 

Sclerotic 393,  354 

Scolopendra 106 

Scorpion 120,  311,  322,  392,  8O 

jaws  of 261 

respiration  in 322 

Scorpionida 120 

Scutes 344 

Scyphozoa 69,  72 

Sea  anemone 75,  387 

see  Polyp. 

Sea  blubber 73 

Sea  butterfly 130 

75,  35 

130 

Seal     ......      189,  378,  325,  377 

Sea  lemon 130 

Sea  lily .      <»:> 

Sea  lion 189,  179 

Sea  mat 219 

Sea  moss 220 

Sea  pen 75,35 

Sea  slug 92,  130,  49 

Sea  urchin, 

35,    91,    252,    257,    48,    226,    237 

absorbent  system  of 297 

circulation  in 310 

digestive  cavity  of 278 

digestive  process  of 295 

eyes  of 391 

growth  of 426 

respiration  in 319 


SEA  URCHIN 

shell  of .     .338,  293 

spines  of 294 

teeth  of 265 

Sea  worm 97,  279,  51 

Secreting  membranes 286 

Secreting  organs 330 

Secretion 245,329 

Segmentation 409,  361 

Selection 455,  456,  465,  466 

Self-division 402 

Semicircular  canals       390 

Sensation 242,386 

Sense  organs 416 

Senses 386 

Sensibility 222 

Sensory  nerve 376 

Sepia 135,  1O7" 

Septum 25,  44 

Series 51 

Serosity 302 

Serpent 269,  275,  284,  337,  371 

see  Snake. 

Sertularia 19 

Serum 302 

Setophaga  ruticilla 161 

Seventeen-year  cicada 67 

Sexton  beetles 77 

Sexual  reproduction      ....      402,  403 

Shaft 846 

Shark       .     .     .  147,  148,  268,  388,  406,  122 

egg  of 36O 

endoskeleton  of 846 

muscle  of 364 

scales  of 146,343 

Sheath,  medullary 239 

Sheep 881,401,427 

Shells 337 

Shipworm 127 

Shrew 192,264 

Shrew  mouse 183 

Shrimp 103 

Sight 391 

Silk  secretors       238 

Silkworm 116 

Silpha 117 

Simia  satyrus 188 

Sinus 347 

Siphon 320,  86 

Siphuncle 136 

Sirenia 184 

Size  of  animals 433 

Skeleton 335,337 

of  arthropod 53 

of  vertebrates.    .191,303,309-317 

Skin 333,335 

from  horse's  nostril 291 

muscles 365 

offish     .- 299 

Skull,  bone  of 205 


5io 


INDEX 


SKULL 

formation  of .     .    352 

of  ant-eater 168 

of  babirusa 232 

of  boa  constrictor 235 

of  chimpanzee 189 

of  dog 353,  305-3O7 

of  European 195 

of  horse 308 

of  negro 196 

of  orang-outang 188 

of  rodent 18O 

Sloth 181,  273,  346 

Slug 131,  267,  92 

Smell 388 

Snail 127,  128,  388,  92,  218 

anatomy  of 243 

circulation  in 311 

eyes  of 391 

fresh-water 1O4 

gullet  of 282 

head  of 351 

jaw  of 218 

locomotion  of 371 

nervous  system  of 378 

teeth  of 265,  266 

temperature  of 328 

touch  of 387 

veligerof 372 

see  Gastropoda. 
Snake      ....       154,  155,  354,  422,  429 

circulation  in 313 

heads  of 135 

locomotion  of . 367 

lungs  of 281 

poison  of 334 

respiration  in 326 

teeth  of 156,  269 

tongue  of 263 

touch  of 387 

vertebral  column  of 352 

Snipe 170 

Solaster 90 

Somite 469 

Songsters 176 

Sorex 183 

Sow  bug 104 

Sparrow 176,  160 

Specialization 293 

Species 51,433 

Spelerpes  ruber 130 

Sperm  cell 408 

Sperm  whale 17O 

Sphenoid  bone 353 

Sphinx  ligustri 333 

Sphinx  moth,  anatomy  of.     .     .  114,241 
Spider,  121,  122,  123,  405,  407,  429,  81 ,  223 

alimentary  canal  of 281 

circulation  in 311 

eyes  of 892 


SPIDER 

fang  of 216 

legs  of 371 

locomotion  of 867 

muscle  of 365 

respiration  in 322 

silk  of 334 

Spinal  cord,  of  am phiox us 117 

of  tunicate 116 

Spindle  shell 129,  96 

Spinnerets 121,82,223,238 

Spiracles 108, 321,  276 

Splint  bone 356,  419 

Sponge,  301,  319,  335,  366,  422,  13,  15, 16 

egg  of 405,359 

Spongilla 66 

Spongin 66 

Spoonbill 165 

Sporozoa 61,  7 

Squamata 154 

Squamosal  bones 353 

Squash  bug 112 

Squid 135,367 

see  Cuttlefish. 

Squirrel 191,  328,  374 

flying 370 

Stag 174 

Stapes 390 

Starfish, 

220,  252,  405,  419,  426,  45,  46,  323 

circulation  in 310 

digestive  process  of 295 

eyes  of 391 

locomotion  of 367,  371 

mouth  of 257 

nervous  system  of  ....    378,  33O 

respiration  in 319 

skeleton  of 338 

stomach  of 278 

see  Echinodermata. 

Steganopodes 165 

Stentor 68 

Sternum 354,  151 

Stilt 170 

Stomach,  coats  of 255 

of  horse 251 

oflion 253 

of  mammals 288 

of  porpoise 252 

of  ruminant 254 

of  tunicate 115 

Stork 165 

Stri* 237 

Striated  muscular  fibers         ....  2O9 

Stridula 399 

Striges 173 

Strix  pratincola    . 155 

Strombus 129,  1O3 

Strophocheilus 131,  93 

Struggle  for  existence  ....      438,  453 


INDEX 


Struthio 162,  14O 

Sturgeon      .     .  146,  149,  254,  346,  364,  125 

Stylonychia 441 

Sun  star 90 

Surinam  toad 407 

Survival  of  the  fittest  .     .     .    .     .     .439 

Suture 356 

Swallow 176,  328,  397 

instinct  of 397 

temperature  of 328 

Swan 287 

Sweetbread 331 

Swift 173 

Swimmerets 100,  54 

Sympathetic  and  spinal  nerves      .     .  343 

Synovia 356 

Systematic  circulation 309 

TABANUS  LINEOLA 282 

Tactile  corpuscles 887 

Tamia 82, 250,  37 

Tanager 176 

Tapetum 395 

Tapeworm 82,  250,  37 

Tapir 186,  264,  376 

Tarsus 356 

Taste 388 

Teeth,  of  animals 265 

of  chimpanzee 233 

of  elephant 234 

offish 147 

offrog 152 

ofhorse 185 

ofman 199 

of  ox 186 

of  snake 156 

Telea 116 

Teleostomi 148 

Telson 99 

Temperature 327 

Temporal  bones  ........    353 

Tendon 366 

Tentacles 387 

Tentaculifera 63,  64 

Tent  caterpillar 116 

Terebra  maculata 98 

Terebratulina  septentrionalis     .     .  43,  44 

Teredo 127 

Termes 110 

Tern 170,  151 

Terrapene  Carolina 137 

Testudo 158 

Tetrabranch 135 

Theria 180 

Thoracic  duct 299,  258 

Thorax 289,  354,  285 

Thornback -  ...    149 

Thousand-legged  worm 106 

Thrush 176 

Thumbless  monkey 217 


Thylacinus       181 

Thyone 35 

Thyroid  cartilage 400 

Tibia 372,  356 

Tick 123 

Tiger 190 

Tiger  beetle 117 

Tineids 116 

Tipula 113 

Tissue 229,  20O,  2O1,  2O7 

Toad   ....    153,269,327,328,337,422 

Toes 356,373 

Tongue,  human 225 

Top  shell 1O2 

Tortoise  .     .     157,  269,  284,  372,  137,  312 
see  Turtle. 

Tortoise  shell 344 

Tortoise-shell  butterfly      .....     72 

Toucan 173 

Touch 387 

Trachea 325,  278,  283,  356 

Trachefe 321 

Tribe 51 

Trichia 3 

Trichina  spiralis .84,39 

Tringa  hypoleuca 152 

Tritonia 129, 130,  9O,  297 

Trochanter 372 

Trochelminthes 82,  85 

Trochosphere 422,  371 

Trochus 129 

Trogon 173,  156 

Trout 146 

Trumpet  animal 63 

Trumpet  shell 129 

Tubipora 75,  79,  27 

Tunicates    .     .    .   140,  142,  336,  115,  116 

Turbinares 165 

Turbo 129,  1O2 

Turkey 169,  287,  338 

Turtle     .    157,  158,  262,  269,  406,  407,  136 

beak  of .344 

bones  of 346 

circulation  in 313 

plates  of 844 

respiration  in .     .    326 

see  Chelonia. 

Tusks 186 

Tympanic  bones 353 

Tympanum 390,  349 

Types 49 

Tyrannus 159 

ULNA      355 

Umbo 125 

Umbrella-acaleph 73 

Ungulata 185,873 

Unio 125,127,341,406 

Univalve 320, 341,  297 

Urinator  imber  - 141 


512 


INDEX 


Urochorda 140,142 

see  Ascidian,  Tunicates. 
Urodela 152 

VANE 346 

Vanessa 115,  78 

Variation 427,454 

Vegetative  life 242 

Vegetative  repetition 220 

Veins 309,  865 

Veliger  of  snail 422,  378 

Velum 423 

Velutina,  odontophore  of 287 

Vena  cava 309 

Venous  valves 864 

Venus     .    .    .  * 127 

Vermes ,.     .   82, 433 

see  Earthworm,  Worm. 

Vertebra 848,  352,  304 

Vertebrata  ....    50,  138,  140,  144,  417 

absorbent  system  of 298 

blood  of 301 

brain  of 380 

circulation  in 813,  118 

deglutition  in 274 

exoskeleton  of 343 

eye  of 393 

liver  of 331 

mouth  of 262 

muscle  of 365,  366 

nervous  system  of   ....      139, 379 

number  of 433 

optic  nerve  of 391,  395 

organ  of  hearing  of 389 

respiration  in 324,  325,  326 

teeth  of 346 

see  Bird,  Crocodile,  Fish,  Frog,  Mam- 
malia, Reptilia. 

Vespa 119 

Vespertilio 184 

Vestibule 390 

Villi 293,298 

Vinegar  eel 84 

Viper 155,  134 

Vireo . 168 

Vitality 225 

Vitelline  membrane      ....      403,404 

Vitreous  humor 393,  394 

Viviparous 140 

Voices  of  animals 399 

Volute 129,  1O1 

Volvox 60 

Vorticella 63,  407,  11' 

Vulpes  pennsylvanicus 178 

Vulture       168,  286,  313 


WALKING  STICK 
Walrus    .    .     . 
Warbler 


Warning  coloration       .         ...    429 

Wasp 119, 406 

Water  beetle 384 

Water  boatman 112,  66 

Water  flea 56 

Waxwing 176 

Weasel 190,  177 

Weevil 118 

Whale      ....       182,  183,  251,  388,  401 

bones  of 346 

brain  of 380 

Greenland 418,  171 

intestines  of 292 

mouth  of 264 

sperm     .........       17O 

upper  jaw  of 838 

whalebone  .    .  '  .     .    .      267,  344,  311 
Wheel  animalcule     ...'..    85,  4O 
Whelk     .     .    .   129,  252,  257,  58,  88,  887 
see  Snail. 

White  ants       110 

Wild  bee 220 

Wilson's  petrel 165 

Windpipe 325 

Wingless  flea 113 

Wings 369,  378 

Wing  shell 129,  1O3 

Wolf 190,  176 

Wombat 180 

Woodcock 170 

Wood  louse 104 

Woodpecker 173,  154 

Worm     ....       303,405,422,426,428 

absorbent  system  of 297 

bristle-footed 95 

circulation  in 311 

cuticle  of  - 336 

gastrula  of 36SJ 

locomotion  of 367 

mouth  of    .     . 258 

nervous  system  of 878 

planarian 38 

teeth  of 346 

see  Earthworm,  Leech,  Nereis. 

Wren      . 176 

Wrist 355 

XIPHOSURA 123 

YOLK 403,  404 

ZEBRA 427 

Zoantharia       75 

Zonotrichia  albicollis 16O 

Zooid .     .     .       433,  8O 

Zoology       11 

Zoothamnium '  .      63 

Zygapophyses 849 

Zygmotic  arch 354 


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